Synthesis and 3d printing of photocurable colloids

ABSTRACT

Photocurable colloid binders are provided that overcome deficiencies associated with 3D printing of high molecular weight polymers via VAT photopolymerization. Methods of additive manufacturing are also provided using the binders. The approaches described herein effectively decouple the viscosity-molecular weight relationship by synthesizing and processing photo-reactive aqueous colloids that are sequestered within a photocrosslinkable scaffold. Sequestering polymers within discrete internal phases prevents inter-particle entanglement of the polymer chains, thus ensuring low viscosity. VP of polymer colloids results in a solid green body embedded with high molecular weight polymer particles. A post-processing heated drying step allows the polymers to coalesce and further entangle, forming a semi-interpenetrating network with mechanical performance of the high molecular weight material. The resins can further include inorganic particles such as silica and other ceramics, metal particles, and the like. The coalescence can result in the particles being encapsulated in polymer, yielding unique hybrid materials with tunable properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims the benefit of priorityto, co-pending U.S. application entitled “SYNTHESIS AND 3D PRINTING OFPHOTOCURABLE COLLOIDS” having Ser. No. 17/442,807, filed Sep. 24, 2021,which is the 35 U.S.C. § 371 national stage application of PCTApplication No. PCT/US2020/024793, filed Mar. 26, 2020, where the PCTclaims priority to, and the benefit of, U.S. provisional applicationentitled “SYNTHESIS AND 3D PRINTING OF PHOTOCURABLE POLYMER COLLOIDS”having Ser. No. 62/823,478, filed Mar. 25, 2019, the contents of whichare incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure generally relates to additive manufacturingmethods and compositions.

BACKGROUND

Vat photopolymerization (VP), also termed stereolithography, representsa versatile additive manufacturing (AM) platform, which enables thefabrication of precise and complex geometries that are unachievablethrough conventional polymer processing techniques. However, the printedobjects typically comprise highly crosslinked, brittle polymericnetworks, which severely restrict their utility as functional parts. VPconventionally delivers patterned UV light in a layer-by-layer fashionwith free radical initiated photocuring of liquid precursors. Theresulting three-dimensional objects exhibit superior micron-scaleresolution, isotropic mechanical properties, and surface finish comparedto other AM platforms.^(1,2) The typical maximum VP printable viscosity(

10 Pa·s) dictates the maximum molecular weight of the liquid precursors;the recoating process between the photocuring of each microscale layeris the primary determinant.³ Successful VP printing demands that eachlayer provides sufficient modulus to maintain feature fidelity, and ourpreliminary efforts have identified a necessary storage modulus(typically in the 10⁴-10⁶ Pa range) to ensure feature fidelity in aprinted object.⁴⁻⁶ VP photocuring conventionally employs covalentcrosslinking and high crosslink densities, which result in an imperfectnetwork from low molecular weight precursors, leading to a suitablemodulus but inferior mechanical performance (e.g., elasticity). Linearcopolymerization of monomers in the VP printing environment, whichpotentially reduces crosslink density, fails to attain sufficientmolecular weight in the printer due to atmospheric oxygen inhibition.Thus, current compositions in the literature do not achieve theprerequisite modulus without high concentrations of a crosslinkingreagent.⁷

Compared to other Additive Manufacturing (AM) processes, VatPhotopolymerization (VP) offers superior resolution, accuracy, andsurface finish. Despite these advantages, VP has seen limited industrialadoption for fabricating end-use products due to process-imposedmaterial constraints. Specifically, the need for multiple photo-reactivefunctional groups results in highly cross-linked polymeric networks thatdo not possess the elasticity and toughness to make them viable fordynamic loading.

Core Problem: VP can only process low viscosity resins, whicheffectively prohibits the use of polymers with high molecular weightsand thus limits parts' mechanical properties.

There remains a need for improved 3D printing methods and compositionsand articles made therefrom that overcome the aforementioneddeficiencies.

SUMMARY

Photocurable colloid binders are provided that overcome deficienciesassociated with 3D printing of high molecular weight polymers via VATphotopolymerization. Methods of additive manufacturing are also providedusing the binders. The approaches described herein effectively decouplethe viscosity-molecular weight relationship by synthesizing andprocessing photo-reactive aqueous colloids that are sequestered within aphotocrosslinkable scaffold. Sequestering polymers within discreteinternal phases prevents inter-particle entanglement of the polymerchains, thus ensuring low viscosity. VP of polymer colloids results in asolid green body embedded with high molecular weight polymer particles.A post-processing heated drying step allows the polymers to coalesce andfurther entangle, forming a semi-interpenetrating network withmechanical performance of the high molecular weight material. The resinscan further include inorganic particles such as silica and otherceramics, metal particles, and the like. The coalescence can result inthe particles being encapsulated in polymer, yielding unique hybridmaterials with tunable properties.

In some aspects, a method is provided comprising photopolymerizing aresin composition to form a green body, the resin composition comprisinga polymer colloid comprising a discontinuous polymer phase comprisingpolymer particles and a continuous solvent phase; one or morephotocrosslinkable scaffold precursors; and a photoinitiator; whereinthe green body comprises a photocrosslinked network of the scaffoldprecursors having the polymer particles entrapped and dispersed therein;drying the green body to produce the article, wherein the drying resultsin penetration of the polymer from the polymer particles through thescaffold and coalescence of the polymer between the polymer particles.

Other systems, methods, features, and advantages of resins and additivemanufacturing methods will be or become apparent to one with skill inthe art upon examination of the following drawings and detaileddescription. It is intended that all such additional systems, methods,features, and advantages be included within this description, be withinthe scope of the present disclosure, and be protected by theaccompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be readily appreciatedupon review of the detailed description of its various embodiments,described below, when taken in conjunction with the accompanyingdrawings.

FIG. 1 is a schematic of the synthesis and printing of aqueous polymercolloids according to certain aspects of the disclosure.

FIG. 2 is a schematic of phase elements of polymer dispersion accordingto certain aspects of the disclosure.

FIG. 3 is a schematic of custom scanning-mask projectionvat-photopolymerization (S-MPVP) machine utilized in certain aspects ofthe disclosure.

FIG. 4 is a schematic of Vat photopolymerization printing and post-printprocessing of photocurable latex into semi-interpenetrating polymernetworks (sIPN).

FIG. 5 is a schematic of continuous-phase photocrosslinking strategy tocreate photocurable latex.

FIGS. 6A-6K show investigation of photocurable latex to sIPN approach.FIG. 6A is a schematic of photocurable latex, (FIG. 6D) photocured greenbody, and (FIG. 6G) dried sIPN. (FIG. 6B) TEM of uncured, photocurablelatex spin-casted on grid. Apparent aggregation of particles is artifactof sample preparation. (FIG. 6C) DLS confirms consistent particle sizeand distribution with and without scaffold monomers. (FIG. 6E) TEM ofspin-casted photocured latex in green body state (FIG. 6F) Green bodyG_N{circumflex over ( )}0 across scaffold compositions (FIG. 6H) TEM ofspin-casted photocured latex in dried, IPN state. (FIG. 6I) DMA of sIPNsacross scaffold composition (FIG. 6J) Tensile performance of photocast &dried IPNs across scaffold compositions. (FIG. 6K) Cyclic loading toconfirm elastic deformation and hysteresis (0.2:1 PEGDA:NVP).

FIGS. 7A-7G show Vat photopolymerization of light-scattering latex.(FIG. 7A) Scanning mask projection vat photopolymerization (S-MPVP)enables fabrication of specimens with large footprint and highresolution. (FIG. 7B-FIG. 7D) In-situ computer vision captures actual UVintensity distribution on the resin surface. Clear resins (FIG. 7B)scatter less UV light which results in uniform gradation of intensity onthe resin surface (white□gray) while heterogeneous resins (FIG. 7C),such as the photocurable latex, scatter more UV light and lower the peakintensity distribution in the resin (gray throughout). (FIG. 7D)Normalization reveals non-uniform scattering inside the projection area.(FIG. 7E) Comparing the desired dimension (box) with the square pillarprinted without compensating for the XY-UV scatter highlights that theprinted part exceeds design dimensions and has rounded edges with pooredge definition (FIG. 7F) Compensating for XY-UV scatter throughiterative optimization of projected intensity distribution results inthe fabrication of pillars with improved dimensional accuracy and edgedefinition. (FIG. 7G) Modeling the energy distribution for specimensprinted via scanning process enables control of cure-through anddimensional accuracy by varying scan speed and projection frame rate.The optimized printing parameters selected for this work are predictedto induce a XY-dimensional reduction of 8 μm at a cure depth of 100 μm.Truncating the cure depth by setting the layer thickness to 100 μmresults in a cure profile that is similar to the desired profile.

FIGS. 8A-8G show evaluation of 3D objects printed from latex. Images andperformance of 3D printed photocurable latex objects: 3D printed Schwarzlattice in (FIG. 8A) green body and (FIG. 8B) dried sIPN states, (FIG.8C-FIG. 8F) printed elastomer molds, and (FIG. 8G) impellor casted fromField's metal alloy.

FIG. 9 is a schematic of photocuring of PEGDA and NVP in the continuousphase of SBR latex yields hydrogel embedded with SBR particles.Photorheology elucidates this photocuring behavior as storage modulus(G′ & G″) as a function of UV light exposure. Light on at 30 s mark.

FIG. 10 shows drying of photocured greenbodies yields translucent sIPNnetworks. TEM confirms coalescence of SBR particles entrapped inNVP/PEGDA photocured scaffold.

FIG. 11 shows TEM images of photocured latex greenbody in wet state(k-kit) showing well dispersed particles throughout curing process.

FIG. 12 shows TEM images of photocured latex in dried state (k-kit)showing well dispersed particles with increased radius due topenetration with scaffold. Rod-like objects are fragments that appearedafter drying and are likely from the k-kit body itself.

FIG. 13 is a graph of working curve of the 0.25:1 PEGDA:NVP latexcomposition shows adherence to the Jacobs equation when the layerthickness is <700 μm. The Critical Energy and Depth of penetration werecalculated to be 74 mJ/cm2 and 206 μm respectively

FIG. 14 is a flowchart of a method to determine the process parametersfor printing with the S-MPVP system. First the desired energydistribution (ER) is numerically computed. Then, through the use ofknown parameters (pixel intensity distribution measured via in-situcomputer vision technique and resin curing properties (Ec and Dp), thescan speed (V) and intensity (I) required to fabricate the part areiteratively determined. Additional constrains such as speed limit of thelinear stages (Vlim) and the maximum intensity of the UV lamp aresupplied to ensure the predicted parameters are within achievableranges.

FIG. 15 is an image of VP printed cubes from 0.4:1 PEGDA:NVP latexcomposition for shrinkage analysis. Surfaces ridges are due to parabolicattenuation of light in the z direction, a common artifact for VP partswith large (>10 μm) layer thicknesses. Wet greenbody (left) and driedsIPN (right) states shown. sIPN cubes are clear along axis of direction(top right), confirming a lack of discrete interfaces between layers.

FIG. 16 is a graph of tensile analysis of VP printed tensile specimens(0.4:1 PEGDA:NVP). Confirms consistent tensile behavior between multiplespecimens with all achieving extensibilities above 500%.

FIGS. 17A-17B shows the single-pixel intensity distribution, capturedvia computer vision, is used to compute the overall intensity profilefor any pattern projected on the resin surface. Simulations of theintensity distributions arising from the projection of a simple squarelattice (FIG. 17A) and the Schwarz lattice (FIG. 17B) illustrate thedisagreement between the desired profile and the actual projectedprofile. The scattering at the edges leads (red and green regions in(FIG. 17A) and (FIG. 17B) respectively) to the fabrication of oversizedfeatures with poor edge definition

FIGS. 18A-18B show particle size and evolution of 50 wt % HMA latexduring batch and semibatch (started at 60-min mark) emulsionpolymerization (FIG. 18A) and DLS analysis of particle sizes of latexesacross copolymer compositional range (FIG. 18B).

FIG. 19 . SEC of MMA/HMA copolymers and homopolymers

FIG. 20 is a graph of measured T¬g¬ values (via DSC) for all latexpolymers across compositional range. Close agreement with fox equationpredictions suggests random copolymerization.

FIG. 21 is a graph of photorheology of 20 wt % HMA latex at totalloading of 8:1 Latex:Scaffold. Investigation of varied scaffold monomer(NVP) to crosslinker (MBAM) weight ratios.

FIG. 22 is a graph of photorheology of MMA/HMA latex series with 8:1Latex:Scaffold and 10:1 NVP:MBAm

FIG. 23 is a schematic showing (part A) Graphical representation ofdrying and interpenetrating process from photocured greenbody to sIPNstates. (part B) 20 mm photocured greenbody discs within minutes ofphotocuring (some drying on benchtop during sample preparation). (partC) Discs after drying in vacuo overnight at 80° C. Opacity impliesretention of discrete latex particle domains. (part D) 0 wt % HMA (PMMAhomopolymer) after drying overnight at 160° C.

FIG. 24 is a graph of comparison of measured Tg values (from DSC) forneat latex polymers and their corresponding sIPN's.

FIG. 25 is a schematic photocurable polymer-inorganic hybrid colloidsenable UV-DIW 3D printing of hydrogel green bodies which yieldsemi-interpenetrating polymer network (sIPN) nanocomposites upon dryingaccording to certain aspects of the disclosure.

FIG. 26A is a diagram of how carboxylate functionalization of silicananoparticles enables water dispersibility. FIG. 26B is a plot ofdynamic light scattering confirming size of dispersed nanosilica andpolymer particles in water (1 wt %).

FIG. 27A is a graph of steady-state shear analysis elucidates shearthinning behavior for all colloids with increasing viscosity at highersilica loadings (higher Silica:SBR). Total solids content is constantbetween all samples (40 wt %). FIG. 27B is a graph of viscosity atvarious shear rates as a function of fractional silica in the totalsolids (Silica:SBR), ie. 30:70 Silica:SBR corresponds to 0.3.

FIGS. 28A-28B show plots of the oscillatory rheology experimentselucidate shear-dependent crossovers of storage G′ (

) and loss G″ (

) shear moduli for high-silica hybrid colloids. (FIG. 28A) Strain sweepand (FIG. 28B) stress sweep experiments elucidate critical yield strainsand stress, respectively.

FIGS. 29A-29B show plots of hybrid colloids at (FIG. 29A) 30:70Silica:SBR and (FIG. 29B) 50:50 Silica:SBR exhibit rapid and reversiblecrossovers at low (0.1%) and high (50%) strain amplitudes. Reversibleliquid-solid transitions evident by crossovers of shear storage (

) and loss (

) modulus.

FIG. 30A shows NVP and PEGDA provide photocrosslinkable scaffoldprecursors in the continuous phase of colloids. FIG. 30B is a plot ofphotorheology showing rapid photocuring at both low and high silicacontents. Irreversible solidification evident by increase of shearstorage (

) and loss (

) modulus, with crossovers evident for liquid 10:90 and 0:100 Silica:SBRsamples. FIG. 30C is an image of dried nanocomposite IPN films with0:100, 10:90, 30:70, and 50:50 Silica:SBR, from left to right.

FIGS. 31A-31D show (FIG. 31A) Dynamic mechanical analysis and (FIG. 31B)tensile analysis confirm silica reinforcement of sIPN nanocomposites.(FIG. 31C) Cyclic tensile experiments of 30:70 Silica:SBR at a constant(FIG. 31C) and progressive (FIG. 31D) maximum strain elucidatereversible elongation and permanent set.

FIGS. 32A-32H show SEM analysis of freeze-fractured surfaces of IPNnanocomposites at compositions: (FIG. 32A & FIG. 32E) 0:100 Silica:SBR,(FIG. 32B & FIG. 32F) 10:90 Silica:SBR, (FIG. 32C & FIG. 32G) 30:70Silica:SBR, and (FIG. 32D & FIG. 32H) 50:50 Silica:SBR.

FIG. 33A is an image of DIW-printed nanocomposite sIPN 3D objects from50:50 Silica:SBR photocurable hybrid colloid. FIG. 33B is a graph of thetensile analysis showing comparable performance for DIW-printed dogbones(30:70 Silica:SBR) with x-y layers printed at 0°, 45°, and 90° withrespect to the elongation direction.

DETAILED DESCRIPTION

In various aspects, binders and methods of additive manufacturing usingthe binders are provided that overcome one or more of the aforementioneddeficiencies. This is accomplished by effectively decoupling themolecular weight from the viscosity of the binder by temporarilydispersing the materials (polymers, inorganic materials, and the like)in a colloidal form. The colloidal morphology effectively decouples theviscosity-molecular weight relationship for polymers with thesequestering of macromolecules into discrete nanoscale particles, whichmitigates inter-chain entanglement. The colloidal materials aredispersed in a solvent with photocrosslinkable materials that can bephotocrosslinked to form a scaffold, thereby encapsulating the colloidalparticles. A scaffold provides structure to the green body, which canthen be heated in a subsequent drying step. The drying step results in acoalescence or entanglement of the particles to form a continuous oressentially continuous phase penetrating the scaffold, thereby realizingthe mechanical properties of the high molecular weight material from thecolloid.

Using the colloidal particles as a vat photopolymerization material andprinting platform that employs common polymeric latexes as highmolecular weight, low viscosity precursors to address the VPprintability-performance paradox. Photocrosslinking of water-solublenetwork precursors in the continuous phase forms a tunable scaffold thatsurrounds the latex particles, which yields a robust, freestanding greenbody object with suitable modulus for VP operations. We employunprecedented computer-vision-based process parameter generation in theVP printer that compensates for light scattering by the colloid andenables light-based printing of complex shapes without UV absorbers.Subsequent dehydration of printed green bodies under mild conditionspromotes 3D coalescence of the latex particles throughout the printedscaffold. This strategy forms a semi-interpenetrating polymer network(sIPN) and harnesses the mechanical properties of the dispersed, highmolecular weight polymer without requiring extraordinary polymer thermalstability or disrupting the complex geometric features defined duringthe VP printing process. This leads to 3D printed elastomers thatestablish a new benchmark for performance that approaches bulkelastomeric films.

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, and as such may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. The skilled artisan will recognize many variants andadaptations of the embodiments described herein. These variants andadaptations are intended to be included in the teachings of thisdisclosure.

All publications and patents cited in this specification are cited todisclose and describe the methods and/or materials in connection withwhich the publications are cited. All such publications and patents areherein incorporated by references as if each individual publication orpatent were specifically and individually indicated to be incorporatedby reference. Such incorporation by reference is expressly limited tothe methods and/or materials described in the cited publications andpatents and does not extend to any lexicographical definitions from thecited publications and patents. Any lexicographical definition in thepublications and patents cited that is not also expressly repeated inthe instant specification should not be treated as such and should notbe read as defining any terms appearing in the accompanying claims. Thecitation of any publication is for its disclosure prior to the filingdate and should not be construed as an admission that the presentdisclosure is not entitled to antedate such publication by virtue ofprior disclosure. Further, the dates of publication provided could bedifferent from the actual publication dates that may need to beindependently confirmed.

Although any methods and materials similar or equivalent to thosedescribed herein can also be used in the practice or testing of thepresent disclosure, the preferred methods and materials are nowdescribed. Functions or constructions well-known in the art may not bedescribed in detail for brevity and/or clarity. Embodiments of thepresent disclosure will employ, unless otherwise indicated, techniquesof additive manufacturing, nanotechnology, organic chemistry, materialscience and engineering and the like, which are within the skill of theart. Such techniques are explained fully in the literature.

It should be noted that ratios, concentrations, amounts, and othernumerical data can be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a numerical range of “about 0.1%to about 5%” should be interpreted to include not only the explicitlyrecited values of about 0.1% to about 5%, but also include individualvalues (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%,2.2%, 3.3%, and 4.4%) within the indicated range. Where the stated rangeincludes one or both of the limits, ranges excluding either or both ofthose included limits are also included in the disclosure, e.g. thephrase “x to y” includes the range from ‘x’ to ‘y’ as well as the rangegreater than ‘x’ and less than ‘y’. The range can also be expressed asan upper limit, e.g. ‘about x, y, z, or less’ and should be interpretedto include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ aswell as the ranges of ‘less than x’, less than y’, and ‘less than z’.Likewise, the phrase ‘about x, y, z, or greater’ should be interpretedto include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ aswell as the ranges of ‘greater than x’, greater than y’, and ‘greaterthan z’. In some embodiments, the term “about” can include traditionalrounding according to significant figures of the numerical value. Inaddition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numericalvalues, includes “about ‘x’ to about ‘y’”.

In some instances, units may be used herein that are non-metric ornon-SI units. Such units may be, for instance, in U.S. CustomaryMeasures, e.g., as set forth by the National Institute of Standards andTechnology, Department of Commerce, United States of America inpublications such as NIST HB 44, NIST HB 133, NIST SP 811, NIST SP 1038,NBS Miscellaneous Publication 214, and the like. The units in U.S.Customary Measures are understood to include equivalent dimensions inmetric and other units (e.g., a dimension disclosed as “1 inch” isintended to mean an equivalent dimension of “2.5 cm”; a unit disclosedas “1 pcf” is intended to mean an equivalent dimension of 0.157 kN/m³;or a unit disclosed 100° F. is intended to mean an equivalent dimensionof 37.8° C.; and the like) as understood by a person of ordinary skillin the art. Conversions between units are readily known to those skilledin the art, for example a temperature in ° F. may be converted to anapproximate temperature in ° C. using the well-known formulaT_((° C.))=(T_((° F.))−32)×5/9. Unless otherwise specified, values andmeasurements referred to herein are based on atmospheric pressure (i.e.one atmosphere) and room temperature.

The use of “for example” or “such as” to list illustrative examplesshould not be construed to limit the disclosure or the claims to onlythe listed examples. Thus, “for example” or “such as” can mean “forexample, but not limited to” or “such as, but not limited to” andencompasses other similar or equivalent examples. This is not to meanthat, in some aspects, such illustrative examples may not be preferredaspects under the circumstances.

Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. It will be further understoodthat terms, such as those defined in commonly used dictionaries, shouldbe interpreted as having a meaning that is consistent with their meaningin the context of the specification and relevant art and should not beinterpreted in an idealized or overly formal sense unless expresslydefined herein.

The articles “a” and “an,” as used herein, mean one or more when appliedto any feature in embodiments of the present invention described in thespecification and claims. The use of “a” and “an” does not limit themeaning to a single feature unless such a limit is specifically stated.The article “the” preceding singular or plural nouns or noun phrasesdenotes a particular specified feature or particular specified featuresand may have a singular or plural connotation depending upon the contextin which it is used.

The terms “ambient temperature” and “room temperature,” asinterchangeably used herein, refer to a temperature between about 18° C.and about 30° C. Usually, room temperature ranges from about 20° C. toabout 25° C.

The term “substituted” as used herein, refers to the substitution of onefunctional group for another functional group, e.g. substituting ahydrocarbon group with another group. Groups can include hydrocarbons,hydrogen atoms, halogen atoms such as chlorine, fluorine, bromine, andiodine; halogen atom containing groups such as chloromethyl,perfluorobutyl, trifluoroethyl, and nonafluorohexyl; oxygen atoms;oxygen atom containing groups such as (meth)acrylic and carboxyl;nitrogen atoms; nitrogen atom containing groups such as amines,amino-functional groups, amido-functional groups, and cyano-functionalgroups; sulphur atoms; and sulphur atom containing groups such asmercapto groups.

The term “polymer,” as used herein, is given its ordinary meaning asused in the art, i.e., a molecular structure comprising one or morerepeat units (monomers), connected by covalent bonds. The repeat unitsmay all be identical, or in some cases, there may be more than one typeof repeat unit present within the polymer. If more than one type ofrepeat unit is present within the polymer, then the polymer is said tobe a “copolymer.” It is to be understood that in any embodimentemploying a polymer, the polymer being employed may be a copolymer insome cases. The repeat units forming the copolymer may be arranged inany fashion. For example, the repeat units may be arranged in a randomorder, in an alternating order, or as a block copolymer, i.e.,comprising one or more regions each comprising a first repeat unit(e.g., a first block), and one or more regions each comprising a secondrepeat unit (e.g., a second block), etc. Block copolymers may have two(a diblock copolymer), three (a triblock copolymer), or more numbers ofdistinct blocks.

The term “polymer backbone” refers to the main chain of the polymer andhas one or more types of repeating subunits. One or more side chains canbe attached to the polymer backbone and can have a multitude ofstructures.

The term “branch-chained polymer,” as used herein, indicates a polymerwhich is not linear, that is, one in which the polymer molecule is notcomposed primarily of a single linear chain of monomers linked end toend. Examples of branched-chain polymers include short-chain branchedpolymer; a long-chained branched polymer; a hyper-branched polymer, acyclic polymer; a comb-type polymer; a 3-arm star type polymer, a 4-armstar type polymer, a dendritic polymer, and a combination thereof.Branched-chain polymers differ from cross-linked polymer networks whichtend towards an infinite size having interconnected molecules and whichare generally not soluble. In some instances, branched polymers haveadvantageous properties when compared to analogous linear polymers. Forexample, higher molecular weights of branched-chain polymers can besolubilized more easily than those of corresponding linear polymers.Highly branched polymers include, for example, dendrimers andhyperbranched polymers.

The term “short-chain branched polymer” refers to branched-chainpolymers where the individual polymer chains have about 10 to about 500repeat units. The term “long-chained branched polymer” refers tobranched-chain polymers where the individual polymer chains have morethan about 500 repeat units.

The term “hyper-branched polymer,” refers to polymers or oligomers thathave highly branched, three-dimensional, tree-like structures with aplurality of branching points. Hyper-branched polymers can bedistinguished from star-type polymers which consist of multiple polymeror oligomer chains (three or more chains) extending from a central core.Some hyperbranched polymers, on the other hand, can be characterized byhaving a “nucleus” and multiple generations of branches, e.g. one ormore generations of branches and an outermost generation of branchesthat terminate with end group functionalities. As used herein, “nucleus”refers to a central monomer from which branches extend. In some aspects,a hyper-branched polymers can be monodisperse, having a regular andhighly symmetric branching structure. Such monodispersed hyperbranchedpolymers can be referred to in the art as “dendrimers” or “dendritic”polymers.

The term “cyclic polymer,” as used herein, refers to a polymer whereends of the polymer backbone are covalently joined to form a largecyclic structure.

The term “comb-type polymer,” as used herein, refers to a polymer havingtwo, three, four, or more polymer chains extending from a backbone of ashorter polymer or oligomer.

The term “star type polymer,” as used herein, refers to a polymer havingthree or more polymer or oligomer chains extending from a central coremolecule. Star-type polymers can be characterized by the number of“arms” extending from the central core structure. For example. a “3-armstar type polymer” has three polymer chains extending from the centralcore where a “4-arm star type polymer” has four polymer chains extendingfrom the central core.

As used herein, the term “number average molecular weight” or “M_(n)”refers to the common mean or average of the molecular weights of theindividual polymers. The number average molecular weight of a polymercan be determined by gel permeation chromatography, viscometry(Mark-Houwink equation), light scattering, analyticalultracentrifugation, vapor pressure osmometry, end-group titration, andcolligative properties.

As used herein, the term “weight average molecular weight” or “M_(w)” isan alternative measure of the molecular weight computed by the formula

$M_{w} = \frac{\sum_{i}{N_{i}M_{i}^{2}}}{\sum_{i}{N_{i}M_{i}}}$

where N_(i) is the number of molecules having molecular weight M_(i).

As used herein, the terms “polydispersity” and “polydispersity index”(PDI) refer to the ratio of the weight average to the number average(M_(w)/M_(n)).

Compared to other Additive Manufacturing (AM) processes, VatPhotopolymerization (VP) offers superior resolution, accuracy, andsurface finish. Despite these advantages, VP has seen limited industrialadoption for fabricating end-use products due to process-imposedmaterial constraints. Specifically, the need for multiple photo-reactivefunctional groups results in highly cross-linked polymeric networks thatdo not possess the elasticity and toughness to make them viable fordynamic loading. One problem is that vat photopolymerization requiredlow viscosity resins, which effectively prohibits the use of polymerswith high molecular weights and thus limits parts' mechanicalproperties.

To address this deficiency, this disclosure described an approach tophotopolymer synthesis and printing to effectively decouple theviscosity-molecular weight relationship by synthesizing and processingphoto-reactive aqueous polymer colloids. Sequestering polymers withindiscrete internal phases prevents inter-particle entanglement of thepolymer chains, thus ensuring low viscosity. VP of polymer colloidsresults in a solid, structural green body embedded with high molecularweight polymer particles. A subsequent post-processing heated dryingstep allows the polymer particles to coalesce and further entangle,forming a semi-interpenetrating network with increased mechanicalperformance. In addition to introducing photocuring chemistry to polymerdispersions and emulsions, this work provides an ideal combination oflow viscosity and high strain properties unprecedented in available VPphotopolymers.

Among the seven modalities of additive manufacturing (AM) processes, vatphotopolymerization (VP, also referred to as “stereolithography”)remains a popular choice for fabricating prototypes due to its finefeature resolution, high dimensional accuracy, and excellent surfacefinish. However, despite these advantages relative to other AMprocesses, VP has seen limited industrial adoption for fabricatingend-use products primarily due to the limited diversity and quality ofits processable materials.

These material limitations stem from the requirement that, to beprocessable via VP, the polymeric (or monomeric) material must feature aphoto-crosslinkable site. Typically, the polymeric design integrates aninert core with photo-crosslinkable functional groups such as acrylates,alkenes or epoxies.¹ When photocured, these multifunctionalmonomers/oligomers form highly cross-linked polymeric networks thatexhibit high modulus and brittleness, and do not possess the elasticityand the toughness to make them viable for dynamic loading in manyend-use part applications. The VP process also imposes a rheologicalconstraint on potential resins. In general, 3 Pa·s is the upper limit ofresin zero-shear viscosity for VP processing, as viscous materials candamage intricate features or even dislodge parts from the build platformduring the recoating process.² Recoating highly viscous resins alsocreates considerable production bottlenecks in the layer-wise printingprocess. This viscosity constraint effectively prohibits the use ofpolymers with high molecular weights, and thus limits the range ofmechanical properties of final printed parts.

While commercial photopolymer vendors (e.g., Formlabs, Stratasys) haverecently offered elastomeric photopolymers for their photopolymerizationAM processes, their materials do not offer suitable toughness forend-use application. Carbon's latest polyurethane materials currentlyoffer the maximum commercially available toughness (following a thermalpost-curing process), with tensile strengths ranging from 5-45 MPa andmaximum elongations in the range of 120-300%. However, as this materialrequires blocked isocyanate and amine chemistry for the post-processingstep, the scope of viable polymers is majorly restricted.³ Furthermore,being proprietary materials, end-users cannot tailor the materialproperties for specific applications.

Synthetic strategies are required to maintain the perfect balancebetween precursor molecular weight, viscosity, and final networkcross-link density to create tuneable elastomers. Among the currentsynthetic strategies for elastomers, mechanisms with hydrogen bondingexhibit the highest elongation properties. Patel et al. report thesynthesis of a family of versatile elastomers, capable of elongations upto 1100%, by blending different ratios of epoxy aliphatic acrylates withaliphatic urethane diacrylates.⁴ The high strain capability isattributed to the presence of hydrogen bonds, which act as physicalcrosslinks between the hard domains of the aliphatic urethanediacrylates. Although this system possesses exceptional strainperformance through hydrogen bonding association, this strategy does notcircumvent viscosity issues of higher molecular weight polymers andtherefore cannot provide high elasticity under tensile loading. Also, aswith the system stated before, this system is restricted topolyurethanes and hydrogen-bonding containing materials which severelylimits the library of polymers available to this approach. Expansion ofthe concept to high molecular weight systems will increase the viscosityof the photopolymer system beyond the working limits of most VP systems.

Others have shown the use of thiol-ene chemistry to tune the mechanicalproperties of single phase VP resins. Hoyle, Lee and Roper'sinvestigation of thiol-ene click chemistry, a free-radical couplingprocess, demonstrates that mechanical properties of acrylate networksare tuneable with controlled addition of multifunctional thiols, due tothe uniformity in the network cross-link density.⁵ Ware et al. alsoreport the use of thiol-ene chemistry for the synthesis of programmableliquid crystalline elastomers capable of achieving strains up to 150%.⁶Baudis et al. report the tunability of mechanical properties creatinglinear polymer chains with controlled crosslinking using thiol-enechemistry.⁷ By varying concentrations of diacrylates, dithiols andmonoacrylates, Baudis created parts that exhibit strains in the range of100-400%.

Colloids are a class of heterogenous mixtures comprised of a continuousphase and a dispersed, phase (droplets, particles, etc.) on lengthscales of approximately (0.1-10 μm). Stable colloids do not exhibitflocculation and sedimentation of the dispersed phases. The termemulsion describes a liquid-liquid colloid, whereas “dispersion” or“sol” refers to a solid colloidally dispersed in a liquid.⁹ Polymercolloids (commonly termed “latex”) can exist as either an emulsion ofpolymer in organic solution with water or as a dispersion in which onlythe solid polymer is dispersed in water (formed through emulsionpolymerization or through emulsification with subsequent solventstripping). This work enables the additive manufacturing of any liquidpolymer colloid through photo-reactive chemistry, and thereforeencompasses both polymer emulsions and polymer dispersions. Because allcolloidal curing chemistry occurs at the interface or within theaqueous, continuous phase the presence of organic solvent in thedispersed phase does not affect the applicability of this invention.

Although the techniques for the formation and polymerization of aqueouspolymer colloids has received significant attention¹⁰⁻¹⁸, little hasbeen done to explore the unique synthetic platform of colloidal systemsfor advanced technologies. Most applications of dispersions solely focuson their utilization as low viscosity mixtures for dipping or coatingonto surfaces followed by drying to provide functionality andproperties. As described below, previous literature has investigated theintroduction of new chemistries to latex. However, few examples exist ofcuring chemistry to provide solids directly from the colloidal state andno examples exist for the application of colloidal photocuring toprovide high molecular weight, high performance objects from anyprocessing technique, including AM.

As previously discussed, VP AM requires controllable, photo-reactivechemistry to create 3D objects. The introduction of crosslinkingchemistry in aqueous polymer colloids has been studied. Of note, Schloglet al. provided an early investigation into photo-crosslinking withinaqueous polymer dispersed systems.¹⁹ However, this technique onlyresulted in crosslinking within each dispersed polymer particle (whenperformed directly in the colloidal state), yielding a liquid mixture ofindividually crosslinked particles. This “prevulcanized” liquid productwas merited to providing increased mechanical properties after filmformation and drying. Roesler et al. developed functional polyurethaneaqueous dispersions with acrylate functionality preferentially locatedat the phase interface.²⁰ Estrin et. al. also developed a photocurableform of maleinized polybuadiene rubber through reaction of the maleicanhydride group with hydroxyethyl acrylate and subsequent neutralizationof the carboxylic acid with acrylate amines.²¹ Both of these strategiesprovided photocurability of water-dispersible polymers, but requiredremoval of the aqueous phase before crosslinking. As a result, theseapproaches only provided structural benefits to the final film afterpolymer particle coalescence and are not suitable for directimplementation in VP AM.

The primary use of emulsion in AM is found in the fabrication of highlyporous structures.²²⁻²³ The general technique involves using awater-in-oil emulsion in which the green body consists of a highlycrosslinked, hydrophobic network, effectively surrounding entrappedwater droplets. The water droplets are subsequently evaporated,ultimately yielding a porous, rigid network. Minas et al. demonstratedthe use of Direct-Ink Writing (DIW) with high internal phase emulsions(HIPE's) to create hierarchical porous ceramics²⁴. Sears et al.fabricated highly porous polymeric structures using photo-reactive DIWfabrication with HIPE inks.²² However, the rheological restrictionslimit the resin development for photopolymerizable HIPE's for DIWprocesses. Highly controllable and well defined hierarchical porousstructures have also been fabricated by vat-photopolymerization ofHIPE's.^(23, 25) AM of HIPE's has provided highly complex hierarchicalporous structures, but their contributions are mainly in the areas ofdrug delivery, membrane exchange, cell growth, etc., and there are noreports of fabricating high performance structural components.²⁶ Roh etal explored the DIW AM of dispersions of poly(dimethylsiloxane)(PDMS).²⁷ Liquid, low molecular weight, vinyl-terminated PDMS wasemulsified into an aqueous poly(vinyl alcohol) solution and heated toform crosslinked PDMS particles in dispersed water. Additionalvinyl-terminated PDMS oligomer was then added to the crosslinked PDMSdispersion to form an increased viscosity paste-like “gel” suitable forDIW AM. The printed parts were then heated to promote flowing of theliquid PDMS particles and further crosslinking via the vinyl end groups.This resulted in high resolution objects capable of a maximum strain ofapproximately 140%, however the high viscosity and lack ofphoto-activated curing chemistry restricts this technique to DIW AM,precluding the increased resolution capability afforded by VP.

There is no precedent for the successful implementation of aqueouspolymer colloids in vat-photopolymerization AM to yield high resolution,high performance 3D objects. Lukid et al. jetted liquid rubber latexonto a substrate, mimicking an ink-jetting process, and thermally driedthe latex to form a solid film.²⁸ However, processing challenges (nozzleclogging) limited the fabrication to a monolayer film. Zheng andDeotte's patent describes the broad idea of heterogeneous materialsystems for VP; however, there is no discussion about materialsselection, interfacial interaction, or synthetic strategy—hence itneither challenges nor lends insight into the development aqueouspolymer colloids as described in this work.²⁹ Hsiao et al developedphoto-crosslinkable PDMS “nanoemulsions” through addition ofpoly(ethylene glycol) diacrylate (PEGDA) to the aqueous phase.³⁰ Theemulsions are then printed at temperatures above the “thermogelation”temperature temperature at which the PEGDA inserts into the colloidalinterface, bridging the particles, before being photo-crosslinkedthrough the acrylate end groups. This results in the formation of an“organohydrogel” of crosslinked PEGDA containing droplets of PDMSoligomer. Similar to the work by Roh and coworkers, this study isrestricted to low molecular weight (5 cP) polymer in the internal phase.The result is a low molecular weight, highly crosslinked system with lowmechanical performance. There is no evidence of the ability of theinternal phase polymer to interpenetrate the hydrogel scaffold, furtherrestricting the mechanical performance capability of these materials.Zhang and coworkers demonstrate VP printing of PTFE latex, however theirmethod relies on the thermal stability of PTFE to thermally remove thescaffold and sinter the particles, thus restricting materialselection.³²

To date, a versatile catalogue of engineering polymers does not yetexist to complement the VP AM process. To address this research gap, andthus to enhance the industrial adoption of VP, this work describes asynthetic and printing strategy to effectively decouple theviscosity-molecular weight relationship by synthesizing and processingphoto-reactive aqueous colloids (dispersions or emulsions). Illustratedin FIG. 1 , this strategy explores an approach to the AM of polymercolloids, focusing on the introduction of high molecular weight, highmechanical performance polymers at viscosities suitable for VP. Printingcomponents from these colloids involves a two-step process chain wherein(i) a “green body” is first created via vat photopolymerization. This isaccomplished through introduction of water-soluble scaffolding monomersto the continuous, aqueous phase of a polymer colloid which can bephoto-crosslinked to form a supportive scaffold around the dispersed,high molecular weight internal phase. (ii) A subsequent heated drying ofthe printed green body enables coalescence and penetration of theembedded polymer particles through the scaffold network, resulting in asemi-interpenetrating network with high mechanical performance.Introduction of an array of monomers with varied degree of functionalityprovides the imperative tunability of the scaffold to permitinterpenetration while also providing structural integrity to maintaingreen body shape. Preliminary experiments support the claim that the useof photocurable aqueous polymer colloids enables VP processing of highmolecular weight polymers with tuneable material properties and highmechanical performance without commensurate increases in resinviscosity. While this approach focused on colloidally dispersed SBR forpreliminary investigation, this technique is applicable to any polymerthat can be colloidally dispersed or emulsified in water.

This approach enables printing of high performance, high molecularweight polymers through synthesis of photo-reactive polymer colloids.Polymer dispersions or emulsions from either emulsion-statepolymerization or the emulsification of preformed polymer are viable inthis invention. Although preliminary work focuses on polydieneelastomers, this invention can be applied to any polymer composition,architecture, and molecular weight that can form a stable colloid inwater.

Emulsification of preformed polymers is well-studied an is possiblethrough high-shear mixing of organic polymer solutions with a nonsolvent(such as water) in the presence of polar or ionic stabilizing groups.For cases in which a free surfactant agent is employed, nearly anypolymer backbone can be emulsified into a colloid by thismethod.^(11-13, 17) This is an ideal approach when polymer versatilityis desirable over interface tunability, or for cases in which thepolymer cannot be easily modified.

However, stabilizing functionality can also be covalently incorporatedon the polymer increasing the tunability of the colloidal system. Thiscan be accomplished through copolymerization of monomers with acidicmoieties (in the case of emulsion polymerization of carboxylated-SBR³¹),or through post-polymerization techniques¹⁵. Although this inventionencompasses a wider variety of polymer compositions, alkene-containingpolymers provide an idea combination of mechanical performance (straincapability, elasticity, etc.) and functionalization potential. Shown inSchemes 1 and 2 below, post-polymerization modification of alkenecontaining polymers (including but not limited to homopolymers andcopolymers containing the monomers butadiene, isoprene,dicyclopentadiene (DCPD), ethylidene norbornene (ENB), vinyl norbornene(VNB), and chloroprene) can provide tuneable, pendant ionic or polarchemistry for stabilization of the phase interface, particularly inwater-based systems. This provides a surfactant-free approach toemulsification of preformed polymers and enables tuneable functionalityat the phase interface. Poussard et. al. provides an example of usingthiol-ene post-polymerization modification to provide ionic moieties forcolloidal stabilization.¹⁵

Scheme 1 illustrates one example that has been reduced to practice withR representing any alkyl carbon sequence, however functionalizationstrategies are not limited to these compounds or chemistries. Furtherwork has explored epoxidation of internal alkenes followed bybase-catalysed thiol-epoxy coupling of thiol functional alkyl carboxylicacids. Other possible approaches include the incorporation of otherpolar or ionizable moieties, such as sulfonates, imidazoles, ethers,nitriles, hydrophilic polymer etc.

Scheme 2 illustrates an example that has been reduced to practice.Sulfonation of EPDM is a well-known reaction that efficiently addssulfonate moieties to a variety of EPDM monomer compositions.

For all cases in which ionic functionality is added to a polymer throughan acidic moiety, deprotonation can occur with any basic compound. Suchcompounds include but are not limited to mono- and multivalent salts,hydroxide salts, amine-containing compounds, carbonate salts, hydrides,nitrogenous bases. It is well understood that the base selected fordeprotonation will determine the chemical identity of the ion andtherefore the functionality present at the colloidal interface. It isalso well understood that these ions can be exchanged in a later step toanother cationic species.

For cases in which ionic functionality is provided through theincorporation of basic moieties, protonation to form pendant cations canoccur with any acidic compound. Such compounds include but are notlimited to hydrogen halides, carboxylic acids, sulfuric acid,ammonium-containing compounds, carbonic acids, citric acid, acetic acid,and phosphoric acid. It is well understood that the acid selected forprotonation will determine the chemical identity of the ion andtherefore the functionality present at the colloidal interface. It isalso well understood that these ions can be exchanged in a later step toanother cationic species.

For cases in which ionic functionality is present through groups thatare not significantly responsive to acid-base reactions, it isunderstood that the ions can be exchanged before or after polymermodification to provide the desired counter-ion and functionalitypresent at the colloidal interface.

Photo-crosslinkable moieties can be introduced to the continuous phaseand/or the phase interface of polymer dispersions and emulsions throughtwo main approaches. As a “scaffolding approach”, a combination ofsoluble/miscible monomers or low molecular weight oligomers can be addedto the continuous phase of a dispersion by mixing. Combined with aphoto-initiators, these provide the photo-reactive precursors thatprovide network formation and curing of the continuous phase. The resultis a solid “green body” consisting of discrete high molecular weightdomains supported by a support scaffold network. Mixture of singly andmultifunctional monomers tunes scaffold crosslink density, providing abalance of structural integrity without hindering interpenetration bythe internal phase polymer. Applicable monomers are any water-solublemonomer that undergoes polymerization or coupling in response to aninitiation process. Examples of these monomers include but are notlimited to any water-soluble compounds that contain any amount orcombination of the following moieties: acrylate, methacrylate, epoxide,thiol, alkene, or alkyne. Scheme 3 below provides a few common examplesof viable monomers for this approach but does not encompass allpossibilities due to the versatile nature of this strategy.

In another approach, photo-reactive functionality can be attached to thepolymer in combination with amphiphilic stabilizing moieties to presentreactivity at the phase interface. Described above, this similarchemistry was employed by earlier work²⁰⁻²¹, but only used to formcrosslinked networks after drying and film formation. Therefore use ofionic, photocurable moieties on polymers as a reactive colloidalinterface remains unexplored. Deprotonation of bound acid functionalgroups on the polymer backbone with basic compounds which containreactive or polymerizable moieties, such as aminoacrylates, providesionic, photo-crosslinkable groups at the colloidal interface. Schemes4-6 below provide examples of this approach to reactive colloidalinterfaces, however any polymer containing acidic moieties (throughpost-polymerization modification or emulsion copolymerization) is viablefor this approach. While this reactive ion approach is describedseparately, it is well understood that it can be used in conjunctionwith the scaffolding method to provide further reinforcement of thecured green body as well as strong association between the highmolecular weight, hydrophobic polymer and the scaffold network. It canbe assumed that this interaction will act as a form of a crosslinking inthe final, semi-interpenetrating network to further enhance mechanicalperformance.

In both the scaffolding and reactive ion strategies described above,photo-initiators are necessary to convert the UV radiation from theprinter to initiate chemical crosslinking. For most activated alkenes(acrylates, methacrylates, vinyl pyrrolidinones, etc.) radicalinitiators are necessary. Any radical photo-initiator, Type I or TypeII, known to those knowledgeable in the art is applicable in thisinvention including, but not limited to benzoin ethers benzil ketalsα-dialkyoxyacetophenones, α-hydroxyalkylphenones, α-aminoalkylphenones,acylphosphineoxides, benzophenones/amines, and thioxanthones/amines.Scheme 7 below shows structures of the most common radicalphoto-initiators employed for vat polymerization. In the case of epoxidefunctional monomers and crosslinkers, a photoacid generator ispreferable for reaction. Examples of these initiators include but arenot limited to Lanthanum(III) trifluoromethanesulfonate andtriarylsulfonium hexafluorophosphate salts.

The AM manufacturing cycle involves three important processes, namely(a) Fabrication of green-bodies via VP systems, (b) UV post curing thegreen bodies and (c) Drying the green bodies to create the final partwith the required thermos-mechanical properties.

The schematic of the machine used for converting the idea to practice isshown above. A custom scanning-mask projection vat-photopolymerization(S-MPVP) machine was used with a build size of 250×250 mm and a featureresolution of 150 μm. In the S-MPVP system, the projection devicetraverses over the resin surface while simultaneously projecting acontinuous bitmap movie. The bitmap movie is a 1:1 scale representationof the features in a single slice of the desired part. The scan speed ofthe projection device and the bitmap movie are synchronized to provideeach projected pixel with the necessary energy for curing up to thedesired layer thickness only.

The STL file of a tensile specimen was sliced into bitmaps of 200 μmlayer thickness using Netfabb®. A custom MATLAB program generated amoving mask for each layer and the corresponding scan speed based on theexposure time estimated from the working curves. A glass vat filled withresin was loaded into the build area. Glass slides were attached to thebuild platform to enhance the adhesion between the printed parts and thesubstrate. The projector traversed over the resin surface whileprojecting the moving mask over the resin. Recoating was performed bylowering the build stage into the resin vat. After a brief pause forresin settling, a recoating blade smoothened meniscus over the buildplatform, ensuring a smooth and level resin surface for fabrication ofthe consequent layers. This process continued until the entire part wasfabricated. During fabrication, the linear stages, the projector and therecoating mechanism are actively monitored and controlled using a customLabVIEW program. The printed parts, extracted from the build platform,were rinsed with an appropriate solvent and wiped with Kimwipes™ toremove non-cured material.

While this represents only one of the possible configurations of VPsystems, any of the combinations resulting from the combination ofcurrently listed parameters or future parameters, that result in thecreation of a new VP process, will deliver a green body at the end ofthe VP fabrication process.

Reaction Light Path Light source Mask Environment Top Down Lamp, LED,Static Mask Reactive Laser environments that inhibit polymerization(i.e. oxygen) Bottom Up Ultraviolet (UV) Dynamic Mask Pro- (i.e DMD,polymerization LCOS, LCD) environments (i.e. Argon, Nitrogen)Holographic Any EM spectra Scanning Mask capable of initiating photo-polymerization Special/voxel

Fabricated green bodies maybe subjected to a UV (or suitable EMradiation) treatment to complete the polymerization of the moieties inthe green body. This step controls the mechanical property of the finalpart. Again, rates of post-curing, environment and use of catalysts toaffect the polymer conversion and hence the mechanical properties, fallunder the scope of the post-curing section, even though not explicitlystated by the inventors.

The process of removing the continuous phase solvent (water) isreferenced henceforth as drying. While the use of a vacuum oven is thetraditional method of drying, any other process that results in dryingfalls under the scope of the patent declaration. The drying process maylead to further entanglements of the polymer chains, or set-off asecondary polymerization reaction or terminate a polymerizationoccurring within the part, all of which affect the mechanical propertiesof the final part.

While the primary intent of the disclosure was to facilitate the 3Dprinting of high-performance polymers, it is necessary to point out thatthe material synthesis strategies are equally valid for processes thatdo not require 3D printing i.e. Ultra-fast cure paint (non-drip),sealant, coating etc. In the non-printing applications, photo, thermalor chemical initiators can be used to drive the polymerization, whilestill retaining the properties of the high-performance polymers.

Areas of application of the methods can include, for example, automobileindustry components such as tires, gaskets, etc.; sealants, coatings ina variety of fields, packaging materials, composites, structuralmaterials, and the like.

Aspects of the Disclosure

The present disclosure will be better understood upon reading thefollowing Aspects which should not be confused with the claims. Any ofthe numbered Aspects below can, in some instances, be combined withother aspects described elsewhere herein even though such combinationmay not be expressly disclosed as such herein.

Aspect 1. A method of additive manufacturing of an article, the methodcomprising: photopolymerizing a resin composition to form a green body,the resin composition comprising a polymer colloid comprising adiscontinuous polymer phase comprising polymer particles and acontinuous solvent phase; one or more photocrosslinkable scaffoldprecursors; and a photoinitiator; wherein the green body comprises aphotocrosslinked network of the scaffold precursors having the polymerparticles entrapped and dispersed therein; drying the green body toproduce the article, wherein the drying results in penetration of thepolymer from the polymer particles through the scaffold and coalescenceof the polymer between the polymer particles.

Aspect 2. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise a polymer having a number average molecularweight of about 50 kg mol⁻¹ to about 10000 kg mol⁻¹, about 50 kg mol⁻¹to about 5000 kg mol⁻¹, about 50 kg mol⁻¹ to about 5000 kg mol⁻¹, about100 kg mol⁻¹ to about 5000 kg mol⁻¹, about 100 kg mol⁻¹ to about 2000 kgmol⁻¹, about 100 kg mol⁻¹ to about 1000 kg mol⁻¹, or about 200 kg mol⁻¹,about 500 kg mol⁻¹, about 1000 kg mol⁻¹, or about 1500 kg mol⁻¹.t

Aspect 3. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise a dispersible polymer selected from the groupconsisting of polycarbonates, polymethacrylates, polystyrenes,polyamides, polyurethanes, poly(ethylene terephthalate), poly(lacticacid), poly(glycolic acid), polyhydroxbutyrate, polydioxanones (e.g.,1,4-dioxanone), 6-valerolactone, 1-dioxepanones (e.g.,1,4-dioxepan-2-one and 1,5-dioxepan-2-one), polyesters, poly(ethyleneglycol), poly(ethylene oxides), polyacrylamides, vinyl polymers, silk,collagen, alginate, chitin, chitosan, hyaluronic acid, chondroitinsulfate, glycosaminoglycans, poly(hydroxyethyl methacrylate),polyvinylpyrrolidone, poly(vinyl alcohol), poly(acrylic acid),polyacetate, polycaprolactone, poly(propylene, glycol)s, poly(aminoacids), copoly (ether-esters), poly(alkylene oxalates), polyamides,poly(iminocarbonates), polyoxaesters, polyorthoesters, polyphosphazenes,polypeptides and copolymers, block copolymers, homoploymers, blends andcombinations thereof.

Aspect 4. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise an elastomer.

Aspect 5. The method according to any one of Aspects 1-30, wherein theelastomer is selected from the group consisting of natural rubber,polyisoprene rubber, styrenic copolymer elastomers (i.e., thoseelastomers derived from styrene and at least one other monomer,elastomers that include styrene-butadiene (SB) rubber,styrene-butadiene-styrene (SBS) rubber,styrene-ethylene-butadiene-styrene (SEBS) rubber,styrene-ethylene-ethylene-styrene (SEES) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-isoprene-styrene (SIS) rubber,styrene-isoprene-butadiene-styrene (SIBS) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-ethylene-ethylene-propylene-styrene (SEEPS) rubber, styrenepropylene-styrene (SPS) rubber, and others, all of which may optionallybe hydrogenated), polybutadiene rubber, nitrile rubber, butyl rubber,and olefinic elastomer such as ethylene-propylene-diene rubber (EPDM)and ethylene-octene copolymers, and copolymers and blends thereof.

Aspect 6. The method to any one of Aspects 1-30, wherein the polymerparticles comprise a high T_(g) polymer such as a poly(arylether),polyester, a polyamide, acrylate polymers such as poly(methacrylate) andpoly(methyl methacrylate), or copolymers thereof.

Aspect 7. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise polymers having a T_(g) of about 300° C.,about 250° C., about 200° C., about 150° C., about 100° C., or less.

Aspect 8. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise a polymer having a T_(g) below a thermaldegradation temperature of the photocrosslinked network of the scaffoldprecursors.

Aspect 9. The method according to any one of Aspects 1-30, wherein theparticles have an average diameter of about 50 nm to about 2.5 μm, about50 nm to about 1 μm, about 100 nm to about 1 μm, about 50 nm to about250 nm, about 50 nm to about 500 nm, about 250 nm to about 1 μm, about100 nm to about 250 nm, about 50 nm to about 150 nm, or about 1 μm toabout 2.5 μm.

Aspect 10. The method according to any one of Aspects 1-30, wherein thesolvent phase comprises water or other aqueous solvents, organicsolvents, or a mixture thereof.

Aspect 11. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise alkene containing polymers selected from thegroup consisting of homopolymers and copolymers containing the monomersbutadiene, isoprene, dicyclopentadiene (DCPD), ethylidene norbornene(ENB), vinyl norbornene (VNB), and chloroprene.

Aspect 12. The method according to any one of Aspects 1-30, wherein thepolymer colloid comprises a surfactant.

Aspect 13. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise polymers having pendant acidic functionalitysome of which has been converted to ionic functionality via reactionwith a base such as mono- and multivalent salts, hydroxide salts,amine-containing compounds, carbonate salts, hydrides, and nitrogenousbases.

Aspect 14. The method according to any one of Aspects 1-30, wherein thepolymer particles comprise polymers having pendant basic functionalitysome of which has been converted to ionic functionality via reactionwith an acid such as hydrogen halides, carboxylic acids, sulfuric acid,ammonium-containing compounds, carbonic acids, citric acid, acetic acid,and phosphoric acid.

Aspect 15. The method according to any one of Aspects 1-30, wherein thephotocrosslinkable scaffold precursors comprise crosslinkable groupsselected from the group consisting of hydrogen halides, carboxylicacids, sulfuric acid, ammonium-containing compounds, carbonic acids,citric acid, acetic acid, and phosphoric acid.

Aspect 16. The method according to any one of Aspects 1-30, wherein thephotocrosslinkable scaffold precursors are in the continuous phase.

Aspect 17. The method according to any one of Aspects 1-30, wherein thephotocrosslinkable scaffold precursors are covalently attached to thepolymer particles at an interface between the discontinuous polymerphase and the continuous solvent phase.

Aspect 18. The method according to any one of Aspects 1-30, wherein thephotoinitiator is a suitable ultraviolet free radical photoinitiatorsuch as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), lithiumphenyl-2,4,6-trimethylbenzoylphosphinate (LAP),oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone),2-hydroxy-2-methyl-1-phenyl-1-propanone, 2,4,6-trimethylbenzophenone,4-methylbenzophenone, 2,2-dimethoxy-1,2-diphenylethanone,2-butoxy-1,2-diphenylethanone, 2-(2-methylpropoxy)-1,2-diphenylethanone, benzophenone, 2-alpha hydroxy ketone,other alpha hydroxy ketones, other benzophenone derivatives or mixturesthereof.

Aspect 19. The method according to any one of Aspects 1-30, wherein aweight ratio of the polymer material to the scaffold precursor materialis about 2:1 to about 20:1, about 2:1 to about 5:1, about 4:1 to about5:1, about 5:1 to about 10:1, about 10:1 to about 15:1, or about 15:1 toabout 20:1.

Aspect 20. The method according to any one of Aspects 1-30, wherein aweight ratio of the polymer material to the scaffold precursor materialis optimized to maximize the polymer concentration without disruptingthe colloidal stability.

Aspect 21. The method according to any one of Aspects 1-30, whereindrying the green body is done at a temperature above a glass transitiontemperature of the polymer in the polymer particles.

Aspect 22. The method according to any one of Aspects 1-30, wherein thearticle has porosity of about 100 nm, about 50 nm, about 20 nm, or less.

Aspect 23. The method according to any one of Aspects 1-30, wherein thearticle is transparent and/or translucent.

Aspect 24. The method according to any one of Aspects 1-30, wherein atransmission electron microscopy reveals nanoscale phase separation andan absence of polymer particles after coalescence.

Aspect 25. The method according to any one of Aspects 1-30, wherein thearticle has a T_(g) that is within 25%, within 20%, within 15%, within10%, within 10° C., within 5° C., or within 2° C. of a theoreticalprediction of the Tg using the Fox method and based on random copolymersof the polymer and neat scaffold.

Aspect 26. The method according to any one of Aspects 1-30, wherein thepolymer colloid further comprises inorganic particles; and

wherein the coalescence of the polymer results in the inorganicparticles being encapsulated and dispersed within the polymer.

Aspect 27. The method according to any one of Aspects 1-30, wherein theinorganic particles comprise silica, carbon particles, metal particles,ceramic particles, and the like.

Aspect 28. The method according to any one of Aspects 1-30, whereinceramic particles include alumina, silicon carbide, silicon nitride,barium titanate, or combination thereof.

Aspect 29. The method according to any one of Aspects 1-30, wherein themetal particles include silver, nickel, iron, cobalt, tungsten,molybdenum, other metals, and alloys and mixtures thereof.

Aspect 30. The method according to any one of Aspects 1-30, whereincarbon particles include carbon black, carbon nanotubes, graphene,graphite, and other carbon nanoparticles.

Aspect 31. A resin composition comprising a polymer colloid comprising adiscontinuous polymer phase comprising polymer particles and acontinuous solvent phase; one or more photocrosslinkable scaffoldprecursors; and a photoinitiator;

Aspect 32. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise a polymer having a number averagemolecular weight of about 50 kg mol⁻¹ to about 10000 kg mol⁻¹, about 50kg mol⁻¹ to about 5000 kg mol⁻¹, about 50 kg mol⁻¹ to about 5000 kgmol⁻¹, about 100 kg mol⁻¹ to about 5000 kg mol⁻¹, about 100 kg mol⁻¹ toabout 2000 kg mol⁻¹, about 100 kg mol⁻¹ to about 1000 kg mol⁻¹, or about200 kg mol⁻¹, about 500 kg mol⁻¹, about 1000 kg mol⁻¹, or about 1500 kgmol⁻¹.t

Aspect 33. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise a dispersible polymer selectedfrom the group consisting of polycarbonates, polymethacrylates,polystyrenes, polyamides, polyurethanes, poly(ethylene terephthalate),poly(lactic acid), poly(glycolic acid), polyhydroxbutyrate,polydioxanones (e.g., 1,4-dioxanone), δ-valerolactone, 1-dioxepanones(e.g., 1,4-dioxepan-2-one and 1,5-dioxepan-2-one), polyesters,poly(ethylene glycol), poly(ethylene oxides), polyacrylamides, vinylpolymers, silk, collagen, alginate, chitin, chitosan, hyaluronic acid,chondroitin sulfate, glycosaminoglycans, poly(hydroxyethylmethacrylate), polyvinylpyrrolidone, poly(vinyl alcohol), poly(acrylicacid), polyacetate, polycaprolactone, poly(propylene, glycol)s,poly(amino acids), copoly (ether-esters), poly(alkylene oxalates),polyamides, poly(iminocarbonates), polyoxaesters, polyorthoesters,polyphosphazenes, polypeptides and copolymers, block copolymers,homopolymers, blends and combinations thereof.

Aspect 34. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise an elastomer.

Aspect 35. The resin composition according to any one of Aspects 31-55,wherein the elastomer is selected from the group consisting of naturalrubber, polyisoprene rubber, styrenic copolymer elastomers (i.e., thoseelastomers derived from styrene and at least one other monomer,elastomers that include styrene-butadiene (SB) rubber,styrene-butadiene-styrene (SBS) rubber,styrene-ethylene-butadiene-styrene (SEBS) rubber,styrene-ethylene-ethylene-styrene (SEES) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-isoprene-styrene (SIS) rubber,styrene-isoprene-butadiene-styrene (SIBS) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-ethylene-ethylene-propylene-styrene (SEEPS) rubber, styrenepropylene-styrene (SPS) rubber, and others, all of which may optionallybe hydrogenated), polybutadiene rubber, nitrile rubber, butyl rubber,and olefinic elastomer such as ethylene-propylene-diene rubber (EPDM)and ethylene-octene copolymers, and copolymers and blends thereof.

Aspect 36. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise a high T_(g) polymer such as apoly(arylether), polyester, a polyamide, acrylate polymers such aspoly(methacrylate) and poly(methyl methacrylate), or copolymers thereof.

Aspect 37. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise polymers having a T_(g) of about300° C., about 250° C., about 200° C., about 150° C., about 100° C., orless.

Aspect 38. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise a polymer having a T_(g) below athermal degradation temperature of the photocrosslinked network of thescaffold precursors.

Aspect 39. The resin composition according to any one of Aspects 31-55,wherein the particles have an average diameter of about 50 nm to about2.5 μm, about 50 nm to about 1 μm, about 100 nm to about 1 μm, about 50nm to about 250 nm, about 50 nm to about 500 nm, about 250 nm to about 1μm, about 100 nm to about 250 nm, about 50 nm to about 150 nm, or about1 μm to about 2.5 μm.

Aspect 40. The resin composition according to any one of Aspects 31-55,wherein the solvent phase comprises water or other aqueous solvents,organic solvents, or a mixture thereof.

Aspect 41. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise alkene containing polymersselected from the group consisting of homopolymers and copolymerscontaining the monomers butadiene, isoprene, dicyclopentadiene (DCPD),ethylidene norbornene (ENB), vinyl norbornene (VNB), and chloroprene.

Aspect 42. The resin composition according to any one of Aspects 31-55,wherein the polymer colloid comprises a surfactant.

Aspect 43. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise polymers having pendant acidicfunctionality some of which has been converted to ionic functionalityvia reaction with a base such as mono- and multivalent salts, hydroxidesalts, amine-containing compounds, carbonate salts, hydrides, andnitrogenous bases.

Aspect 44. The resin composition according to any one of Aspects 31-55,wherein the polymer particles comprise polymers having pendant basicfunctionality some of which has been converted to ionic functionalityvia reaction with an acid such as hydrogen halides, carboxylic acids,sulfuric acid, ammonium-containing compounds, carbonic acids, citricacid, acetic acid, and phosphoric acid.

Aspect 45. The resin composition according to any one of Aspects 31-55,wherein the photocrosslinkable scaffold precursors comprisecrosslinkable groups selected from the group consisting of hydrogenhalides, carboxylic acids, sulfuric acid, ammonium-containing compounds,carbonic acids, citric acid, acetic acid, and phosphoric acid.

Aspect 46. The resin composition according to any one of Aspects 31-55,wherein the photocrosslinkable scaffold precursors are in the continuousphase.

Aspect 47. The resin composition according to any one of Aspects 31-55,wherein the photocrosslinkable scaffold precursors are covalentlyattached to the polymer particles at an interface between thediscontinuous polymer phase and the continuous solvent phase.

Aspect 48. The resin composition according to any one of Aspects 31-55,wherein the photoinitiator is a suitable ultraviolet free radicalphotoinitiator such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide(TPO), lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP),oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone),2-hydroxy-2-methyl-1-phenyl-1-propanone, 2,4,6-trimethylbenzophenone,4-methylbenzophenone, 2,2-dimethoxy-1,2-diphenylethanone,2-butoxy-1,2-diphenylethanone, 2-(2-methylpropoxy)-1,2-diphenylethanone, benzophenone, 2-alpha hydroxy ketone,other alpha hydroxy ketones, other benzophenone derivatives or mixturesthereof.

Aspect 49. The resin composition according to any one of Aspects 31-55,wherein a weight ratio of the polymer material to the scaffold precursormaterial is about 2:1 to about 20:1, about 2:1 to about 5:1, about 4:1to about 5:1, about 5:1 to about 10:1, about 10:1 to about 15:1, orabout 15:1 to about 20:1.

Aspect 50. The resin composition according to any one of Aspects 31-55,wherein a weight ratio of the polymer material to the scaffold precursormaterial is optimized to maximize the polymer concentration withoutdisrupting the colloidal stability.

Aspect 51. The resin composition according to any one of Aspects 31-55,wherein the polymer colloid further comprises inorganic particles; and

wherein the coalescence of the polymer results in the inorganicparticles being encapsulated and dispersed within the polymer.

Aspect 52. The resin composition according to any one of Aspects 31-55,wherein the inorganic particles comprise silica, carbon particles, metalparticles, ceramic particles, and the like.

Aspect 53. The resin composition according to any one of Aspects 31-55,wherein ceramic particles include alumina, silicon carbide, siliconnitride, barium titanate, or combination thereof.

Aspect 54. The resin composition according to any one of Aspects 31-55,wherein the metal particles include silver, nickel, iron, cobalt,tungsten, molybdenum, other metals, and alloys and mixtures thereof.

Aspect 55. The resin composition according to any one of Aspects 31-55,wherein carbon particles include carbon black, carbon nanotubes,graphene, graphite, and other carbon nanoparticles.

Aspect 56. An article made by photopolymerizing a resin compositionaccording to any one of Aspects 31-55.

Aspect 57. An article made by a process according to any one of Aspects1-30.

EXAMPLES

Now having described the embodiments of the present disclosure, ingeneral, the following Examples describe some additional embodiments ofthe present disclosure. While embodiments of the present disclosure aredescribed in connection with the following examples and thecorresponding text and figures, there is no intent to limit embodimentsof the present disclosure to this description. On the contrary, theintent is to cover all alternatives, modifications, and equivalentsincluded within the spirit and scope of embodiments of the presentdisclosure.

Example 1. 3D Printing Latex: A Route to Complex Geometries of HighMolecular Weight Polymers

This example describes concurrent photopolymer and VP system use ofpolymeric colloids (latexes) that effectively decouple the dependency ofviscosity on molecular weight. Photocrosslinking of a continuous-phasescaffold, which surrounds the latex particles, combined with in-situcomputer-vision print parameter optimization, which compensates forlight scattering, enables high-resolution VP of high molecular weightpolymer latexes as particle-embedded green bodies. Thermalpost-processing promotes coalescence of the dispersed particlesthroughout the scaffold, forming a semi-interpenetrating polymer network(sIPN) without loss in part resolution. Printing a styrene-butadienerubber (SBR) latex, a previously inaccessible elastomer composition forVP, exemplified this approach and yielded printed elastomers withprecise geometry and tensile extensibilities exceeding 500%.

Experimental

Materials

Styrene-butadiene rubber (SBR) latex (Rovene 4176) was generouslydonated by Mallard Creek Polymers Inc. The latex has a solids content of50 wt %, a particle diameter range of 120-170 nm, and a viscosity of 400cps as measured by the manufacturer. The SBR copolymer was approximately50/50 by weight styrene and butadiene with a low level of carboxylicacid monomer neutralized with ammonia to provide colloidal stability.The polymer contains a high insoluble (gel) content from thepolymerization process due to intra-particle crosslinking during thepolymerization process. 1-vinyl-2-pyrrolidinone (NVP) and poly(ethyleneglycol) 575 g/mol (PEGDA 575), anddiphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) were purchasedfrom Millipore Sigma and used as received.

TABLE 1 (Thermo)mechanical data for sIPN's and neat SBR from latexScaffold Scaffold Plateau Strain Stress Stress Composition Loading G′ @@ @ NVP:PEGDA SBR:Scaffold (GN0) Tg Break Break 100% (wt:wt) (wt:wt)(kPa) (° C.) (%) (MPa) Strain 1:5 200 16 360 5.4 1.2 1:1 4:1 50 24 4205.3 1.1 2.5:1   4:1 30 9 500 5.5 1.1 5:1 4:1 8 18 540 5 1 Neat SBR 1:0 —5 590 2 0.5

Photocurable Latex Preparation

As a standard example (4:1 polymer:scaffold, 5:1 NVP:PEGDA), 50 mg TPOphotoinitiator was added to a 20-mL vial followed by the addition of 0.5g NVP. The photoinitiator was fully dissolved via vortex mixing beforeaddition of PEGDA and further mixing. The monomer/photoinitiatorsolution was then added via dropwise addition to 5 g latex stirringrapidly in a separate 20-mL vial. The photocurable latex was thenvortexed for 30 s to ensure complete mixing. For print-scalepreparation, a similar method was used, employing a 1 L beaker andmechanical stirrer during the monomer/photoinitiator addition to 400 gof latex.

Analytical Methods

Dynamic light scattering (DLS) measurements were conducted with aMalvern Zetasizer Nano, reporting intensity distributions. DLS sampleswere prepared with latex which was diluted to 1 wt % solids withdeionized water to minimize particle-particle interactions.Photorheology was performed on a TA Instruments DHR-2 equipped with aSmartSwap® UV assembly with 20-mm aluminum upper plate, 20-mm quartzlower plate, and Omnicure S2000 high-pressure mercury light source(320-500 nm filter). UV intensity was measured with a Silverlineradiometer and 20 mm sensor attachment for the quartz parallel plate.Data was gathered at a 500 μm gap, 0.2% strain, and 1 Hz. UV radiationwas applied at an intensity 200 mW/cm² for 150 s after a 30 s delay. Therheometer was set to maintain 0 N axial force within a ±1 N tolerancethrough slight adjustments in gap size. Samples were run under airwithout purge of inert gas. All samples were run in triplicate to ensureconsistency and reproducibility of this technique. Plateau storagemoduli values were calculated from the last 20 s of the G′ curve; modulicrossover (G′/G″) values were determined using the dedicated feature inTA Instruments TRIOS software. Gel fractions were determined as thedifference in dry weight before and after extraction and averaged overthree replicates. Density measurements were conducted with a specificgravity kit and balance according to a previously reported procedure.¹⁶Dynamic mechanical analysis (DMA) was performed on a TA Instruments Q800Dynamic Mechanical Analyzer in tension mode at 1 Hz frequency, 0.2%strain amplitude, and a heating rate of 3° C./min-140 to 150° C. Tensileexperiments were performed on an Instron 5500R tensile tester at astrain rate of 5 mm/min at 23° C. Measurements of engineering stress at100% strain in addition to engineering stress and strain at break arereported. Hysteresis experiments were performed on the same instrumentfrom 0-300% strain at a strain rate of 20% strain/min.

Preparation of TEM Samples

Transmission electron microscopy (TEM) was performed in the dried stateon spin-coated TEM grids (Formvar/Carbon 200 mesh, copper) and in liquidstate via BioMatek k-kits with a 200 nm channel height, purchasedthrough Ted Pella. Imaging was performed with both a Philips EM420 (120kV acceleration voltage) and a JEOL 2100 (200 kV acceleration voltage)for the dry and liquid-state samples, respectively.

Spin-Casted & Photocured TEM Grids

Photocurable latex (4:1 polymer:scaffold, 5:1 NVP:PEGDA) was prepared asdescribed above, using 1 wt % diluted latex in lieu of 50 wt % to aidimaging. Pure latex (without monomer and photoinitiator) at 1 wt %dilution was also prepared for comparison. 70 μL of each latex samplewas then applied to the surface of a TEM grid and spin coated at 4000rpm for 20 s. For photocured samples (green body akgnd IPN state), UVirradiation was applied via an Omnicure S2000 (details above), at 10%shutter for 5 s. IPN state samples were placed in a vacuum oven at 65°C. for 12 h. All other samples were mounted onto a glass slide andplaced into a sealed centrifuge tube with water-saturated kim wipes tominimize drying prior to imaging. TEM imaging occurred promptly aftersample preparation. The wet samples were placed directly into the TEMsample load lock to rapidly dry immediately before insertion into theinstrument and imaging. The dried (IPN state) sample was imaged afterdirectly after removal from vacuum oven.

Liquid-Cell (k-Kit) TEM (Wet-State)

Photocurable latex (4:1 polymer:scaffold, 5:1 NVP:PEGDA) was prepared asdescribed above, using 1 wt % diluted latex in lieu of 50 wt % to aidimaging and a neat 1 wt % without added monomer and photoinitiator forcomparison. Each sample was loaded into the k-kit according to amodified version of the procedure provided with the k-kit toolkit andemploying a stereoscope for visualization of the process. 0.2 uL oflatex was placed on the sample loading stage. The k-kit channel endswere opened via removing the sealing tips at each end, then one end ofthe channel was dipped into the sample droplet, ensuring contact viastereoscope observation. The ends of the k-kit body were cleaned usingpolypropylene swabs, followed by sealing of each end with Hysol 1 C highvacuum sealant. The k-kit was allowed to sit for 1 h at atmospherictemperature and pressure to allow hardening of the sealant beforemounting into the provided copper grid holder with the supplied epoxy.The k-kit was then placed in a vacuum oven at 15 mmHg and roomtemperature to accelerate curing of the sealant and mounting epoxy.Finally, the sample was covered in aluminum foil and stored in a cool,dark place overnight to allow full curing of the sealant.

Preparation of DMA Samples

Greenbody discs cured via photorheology as described previously. Thegreenbody discs were then dried in the vacuum oven at 65° C. for 12 h,extracted in acetone for 12 h and subsequently dried in vacuo at roomtemperature for an additional 12 h. Rectangular specimens were cut fromthe IPN disc films and analyzed directly.

Preparation of Dogbones from Photocast Films for Tensile Analysis

Photocurable latex was prepared as described above. 3 g of each samplewas placed into a glass petri dish (9 cm diameter) and irradiated for 30s on each side with a belt-fed photocuring system (LC-6B) from Fusion UVSystems Inc equipped with a 100 W bulb. The films were subsequentlydried in vacuum oven at 65° C. for 12 h to allow drying and particlecoalescence. The IPN films were then removed from the petri dish andextracted in acetone for 12 h before another drying step in vacuum ovenat room temperature for 12 h to remove acetone. Dogbones were then cutfrom the dried and extracted films using a Pioneer-Dietecs ASTM D-638-Vdie and analyzed directly.

Vat Photopolymerization of Latex

Scanning Mask Projection Vat Photopolymerization Apparatus (S-MPVP)

A custom S-MPVP apparatus was used for specimen fabrication.³⁰ Theapparatus comprises a high-resolution projector with 1080p TexasInstruments DMD (0.65″). The projector is illuminated by Dymax Bluewave75 spot-cure lamp with a broad-spectrum emission in the range of 300-500nm. The intensity on the projection plane, measured with a 365 nmradiometer (xx name), is 2.4 mW/cm². Using imaging and conditioningoptics (DLIinnovations-DLP6500), the projection area and projected pixelsize on the focal plane were measured to be 61×34 mm and 31 μmrespectively. The projector is fixed on cross-mounted X-Y linear stages(ZABER: A-LST0500A-E01) to enable continuous scanning in the X-Y plane.A build stage, additively manufactured using filament extrusion of ULTEM9085, is mounted to a high-resolution Z-stage (ZABER: A-LST0250A-E01). Aglass slide (Corning 294775X50) was mounted to the top surface of thebuild platform to ensure a smooth build surface and good adhesion withthe printed part. A custom glass vat (150×150×40 mm) was manufacturedfor containing the photocurable latex. A recoating blade, mounted to acustom linear actuator, was directly mounted to the X-Y gantry, enablingthe control over recoating speed and recoating depth. The mechatronicelements and the projection were controlled using a custom LABVIEWprogram.³⁰

The scanning mask projection apparatus was used in this work because itenables the fabrication of large area parts with high-resolution throughthe use of the unique scanning process. Instead of projecting a staticframe on the resin surface, the S-MPVP system projects a movie and scansacross the resin surface simultaneously. During the fabrication process,a bitmap corresponding to the layer to be printed is sliced into smallerprojection rows. Each row is then split into multiple static frames suchthat when they are played as a movie, the entire row is projectedwithout loss of information. The speed at which the movie is played isrelated to the scan speed through the S-MPVP model. Through the use of acustom rendering program, the movie is created and played real-timewhile the projector is scanning across the resin surface. This processis repeated for all the scanning rows and all layers if the part to befabricated. The synchronization of the movie and the scanning processare carefully monitored to ensure consistent part fabrication.

Working Curve Generation

30 mL of photocurable latex was transferred into a glass petri dish andplaced in the focal plane of the projector. A test feature (20×20 pix)was projected on the resin surface with exposure times of 5, 6, 7, 8,10, 11, 12 and 18 seconds. The resulting cured specimens were rinsedwith water and UV postcured (Melodysusie 36 W, 365 nm) for 10 minuteseach side. The thickness of the specimens was plotted against exposureto generate the working curve to compute the values for Critical energyand Depth of penetration (resin curing parameters) (FIG. 13 ).¹

Printing Parameter Generation with S-MPVP Model

The intensity distribution, resulting from the projection of a 1-pixelwide line on the resin surface was captured with an embedded digitalcamera (Logitech C920) and processed using a custom computer vision(MATLAB) program. Using a depth of penetration (D_(p)) and criticalenergy (E_(c)), computed via working curve, of 206 μm and 74 mJ/cm²respectively, the S-MPVP model was applied to the test specimen.³⁰

The reference energy distribution required to fabricate an accurate testspecimen was numerically determined by setting all the energy levelsequal to the resin's E_(c). Then, the bitmap pattern corresponding to alayer to be fabricated was fed into the irradiance model (Equation 2)and the actual intensity distribution on the resin surface was computed.Using an algorithm to iteratively select the exposure time andprojection intensity, the cured specimen dimensions were simulated withthe S-MPVP model and multiple energy distributions were generated. Thesimulated cure dimensions were then compared against the desiredspecimen dimension. The combination of exposure time and intensity thatresulted in fabrication of feature with error <10 μm was selected forspecimen fabrication. The flowchart for the process parameteroptimization is shown in FIG. 14 .

For fabrication of specimens in the static mode, the optimizationalgorithm was modified as per Scheme 8. Through iterative selection ofexposure time and grayscaling ratio, the cured specimen dimensions weresimulated with the S-MPVP model. The combination of process parametersthat resulted in the fabrication of features with errors <10 μm wasselected for specimen fabrication.

Specimen Fabrication Via Static MPVP

Autodesk Netfabb was used to slice the STL file of the Schwarz lattice(pore size of 5 mm) and the impellor molds into 100 μm layers. Thelayers were then converted into bitmap images with a resolution of 801DPI. Photocurable latex was transferred into the resin vat and the buildplatform was lowered 100 μm (layer thickness) into the resin. Theprojector, while remaining stationary above the build platform,projected bitmap patterns corresponding to each layer for an exposuretime of 8 seconds. The first layer was exposed 3 times to ensure goodadhesion with the glass slide. The build platform was then lowered intothe resin to agitate the latex and prevent evaporation of water. Afteraccounting for the layer thickness, the build platform was raised to theappropriate height for recoating. A recoating blade traversed across theprinted tensile specimen to enforce deposition of a uniform layer ofuncured resin. The recoating speed was controlled to 5 mm/s to preventdislodging of printed specimen. The projection and recoating cycles wererepeated until complete fabrication of the part. Printed greenbodieswere removed from the build platform and cleaned thoroughly with waterto remove uncured resin. Cleaned greenbodies were UV-postcured for 10minutes (each side).

Specimen Fabrication Via Scanning Mask Projection VatPhotopolymerization (S-MPVP)

Autodesk Netfabb was used to slice the STL file of the tensile specimens(modified ASTM-638 V) into 100 μm layers. The layers were then convertedinto bitmap images with a resolution of 801 DPI. Photocurable latex wastransferred into the resin vat and the build platform was lowered 100 μm(layer thickness) into the resin. The projector scanned across the resinsurface with a scan speed of 4.286 mm/s, while simultaneously projectingthe generated bitmap layers as a movie with a frame rate of 135frames/second. The first layer was exposed three consecutive times toensure good adhesion with the glass slide. The build platform was thenlowered into the resin to agitate the latex and prevent evaporation ofwater. After accounting for the layer thickness, the build platform wasraised to the appropriate height for recoating. A recoating bladetraversed across the printed tensile specimen to enforce deposition of auniform layer of uncured resin. The recoating speed was controlled to 5mm/s to prevent dislodging of printed specimen. The projection andrecoating cycles were repeated until complete fabrication of the part.Printed greenbodies were removed from the build platform and cleanedthoroughly with water to remove uncured resin. Cleaned greenbodies wereUV-postcured for 10 minutes (each side).

Post-Processing of 3D Printed Objects

Post-cured specimens were transferred onto Teflon sheets and placed in avacuum oven, preheated to 45° C. The specimens were placed under vacuum,3 in. Hg, for 48 hours. The pressure inside the vacuum chamber wasslowly equilibrated over 10 minutes and the dried parts were removed forimaging and mechanical testing.

Results and Discussion

Synthetic Design of Photocurable Polymer Latexes

FIG. 4 illustrates VP of photocurable polymeric latexes to print highmolecular weight polymers. Facile addition of network precursors andphotoinitiator to the continuous, aqueous phase of the latex enablesphotogeneration of the supporting scaffold. UV exposure during printinginitiates photocrosslinking of these precursors to form a supportingscaffold around the latex particles, which yields a freestanding “greenbody” hydrogel embedded with high molecular weight latex particles.Subsequent drying and annealing of the green body enables the polymericparticles to diffuse and coalesce throughout the printed object. Theresulting sIPN consists of two discrete components: (1) aphoto-crosslinked scaffold network, which serves to design the 3D shapeof the object, and (2) an entangled, high molecular weight latexpolymer, which dictates mechanical performance of the final printedobject. This strategy is suitable for any polymeric colloid; however,our initial focus on elastomers provides a convincing example of theimportance of high molecular weight to attain high mechanicalperformance. Emulsion polymerization commonly affords styrene-butadienerubber (SBR) latexes and represents a pervasive industrial elastomerotherwise unavailable for AM.

FIG. 4 shows a schematic of Vat photopolymerization printing andpost-print processing of photocurable latex into semi-interpenetratingpolymer networks (sIPN). Photocrosslinking of scaffold molecules in thecontinuous phase of latex entraps polymer particles into a solid greenbody. Drying of the greenbody enables the polymer within the particlesto penetrate the scaffold and coalesce, harnessing the mechanicalproperties of the latex polymer.

A suitable scaffold must meet three basic criteria: (i) scaffoldmonomers and photoinitiators must not disrupt the latex stability, (ii)scaffold monomers must rapidly photocure into a robust network capableof supporting colloidal particles in a 3D design with sufficient modulusfor printing, and (iii) scaffold composition must enable bothprintability (curing kinetics, green body strength) and part performance(tensile strength, elasticity). Addressing each criterion is critical toenable robust green bodies, which maintain complex geometric featuresduring printing, and ensure desired performance upon particlecoalescence and drying.

Illustrated in FIG. 5 , the combination of N-vinyl pyrrolidinone (NVP)and poly(ethylene glycol) diacrylate (PEGDA) served as a suitablescaffold monomer and crosslinker, respectively, and this combinationallowed VP-printed SBR latexes. Dynamic light scattering (DLS), (FIG.6C) confirmed that the scaffold monomers did not deleteriously influenceSBR particle size or particle size distribution. Photorheologicalmeasurements demonstrated the potential for efficient photocuring anddesirable green body storage moduli as a function of UV light exposure(FIG. 9 ). The plateau shear storage modulus (G_(N) ⁰) relates to theM_(c) of the photocrosslinked network⁸ and ensures structural rigidityof the green body as a function of monomer composition, as shown in FIG.6F. G_(N) ⁰ increased significantly (8-200 kPa) with an increase in theconcentration of the PEGDA crosslinker, i.e., higher weight ratios ofPEGDA:NVP. Tuning this ratio enabled optimization of both printing(higher G_(N) ⁰ for structural fidelity of green bodies) and final sIPNmechanical performance (lower G_(N) ⁰ for better tensile properties postdrying). Considerations of colloidal stability restricted total scaffoldloading (SBR:scaffold) to more narrow compositional ranges and a 4:1ratio (80 wt. % SBR and 20 wt. % scaffold) was deemed optimal for thesestudies.

FIG. 5 is a schematic of continuous-phase photocrosslinking strategy tocreate photocurable latex. Incorporation of n-vinyl pyrrolidinone (NVP)and poly(ethylene glycol) diacrylate (PEGDA) into continuous-phaseenables photo-activated crosslinking and solidification of liquid latex.

Drying the photocured green bodies in vacuo changed the film appearancefrom opaque white to translucent (shown in FIG. 6D & FIG. 6G), which wasconsistent with the loss of discrete, light-scattering nanoscale domainsdue to particle coalescence. Furthermore, their mechanical strengthincreased substantially from a soft, fragile green body hydrogel toductile elastomers. FIG. 6J illustrates the effect of scaffoldcomposition on tensile behavior; in particular, sIPN's exhibit a higherultimate stress and lower ultimate strain at higher scaffold crosslinkdensities (increased PEGDA:NVP). Previous literature examples ofelastomer-based IPNs are similar as a function of both elastomerconcentration and network crosslink density.²⁵⁻²⁷ Moreover, a morehighly crosslinked scaffold will presumably decrease the particles'ability to diffuse and coalesce, leading to less extensibility. sIPNs atthe lowest scaffold crosslink density (lowest PEGDA:NVP wt. ratio)achieved strains exceeding 500% and fully reversible deformation overfive cycles at 300% strain (FIG. 6K). It is important to note thatcrosslinked scaffold controls (without latex) were too brittle andprevented tensile specimen preparation, which further suggested that thescaffold serves as a structural template for the printed 3D shape; theinterpenetrating, high molecular weight SBR polymer dominates theultimate mechanical properties. Thus, VP of latexes enables printing oflow viscosity colloids that subsequently manifest mechanical performanceof the high molecular weight polymers and, consequently, address theparadox of printability and performance for VP.

FIGS. 6A-6K show investigation of photocurable latex to sIPN approach.(FIG. 6A) photocurable latex, (FIG. 6D) photocured green body, and (FIG.6G) dried sIPN. (FIG. 6B) TEM of uncured, photocurable latex spin-castedon grid. Apparent aggregation of particles is artifact of samplepreparation. (FIG. 6C) DLS confirms consistent particle size anddistribution with and without scaffold monomers. (FIG. 6E) TEM ofspin-casted photocured latex in green body state (FIG. 6F) Green bodyG_(N) ⁰ across scaffold compositions (FIG. 6H) TEM of spin-castedphotocured latex in dried, IPN state. (FIG. 6I) DMA of sIPNs acrossscaffold composition (FIG. 6J) Tensile performance of photocast & driedIPNs across scaffold compositions. (FIG. 6K) Cyclic loading to confirmelastic deformation and hysteresis (0.2:1 PEGDA:NVP).

Transmission electron microscopy revealed morphological transitionsacross the entire process from photocurable colloids to sIPNs.Spin-casted diluted latex samples (1 wt. % solids) onto TEM gridsenabled imaging of the latex particles. Photocuring and subsequentdrying of these grids facilitated analysis of the green bodies andsIPN's, respectively. In the colloid precursor (FIG. 6B), excellentcontrast existed between the SBR particles and the scaffold monomers.TEM provided particle diameters that agreed well with DLS measurementsin both the absence and presence of scaffold monomers, approximately 150nm. The spin-coating process partially dries the samples, whichpresumably induces particle aggregation; DLS (FIG. 6C) and wet-cell TEM(FIG. 11 & FIG. 12 ) images confirm well-dispersed particles in thecolloid precursor. Photocuring a spun-cast film on a TEM grid confirms acontinuous scaffold film embedded with SBR particles (FIG. 6E).Particles were only located within this film, suggesting the networkscaffold efficiently entraps the colloidal particles. After waterremoval in vacuo, the particles penetrated the scaffold and coalesced,and the loss of spherical shape and nanoscale phase separation supportedthis mechanism, as depicted in FIG. 6H & FIG. 10 .

IPNs (containing two intertwined polymeric networks) and sIPNs(containing a single crosslinked network and non-crosslinked polymer)are widely recognized for their unique morphology and (thermo)mechanicalproperties.²⁸ Due to a high degree of network mixing, sIPNs commonlyexhibit shifting of their component glass transition temperatures(T_(g)) to a single intermediate value, as predicted using the Foxrelationship for random copolymers.^(28,29) Dynamic mechanical analysis(DMA) (FIG. 6I) shows similar behavior for the all dried parts withsingle T_(g)'s at approximately 16, 24, and 18° C. (as measured from DMAtan δ maxima) for the 5:1, 1:1, and 0.2:1 PEGDA:NVP, respectively. Eachintermediate transition temperature favors the T_(g) of the majorcomponent, SBR (80 wt. %), and occurs near the Fox prediction of 16° C.based on the T_(g) of the neat scaffold (1:1 PEGDA:NVP). It is importantto note that these results also suggest an unprecedented methodology forpreparing sIPNs with implications for membrane technologies beyond thescope of additive manufacturing.

Printing Light-Scattering Materials Via Vat Photopolymerization

FIGS. 7A-7G show Vat photopolymerization of light-scattering latex. (A)Scanning mask projection vat photopolymerization (S-MPVP) enablesfabrication of specimens with large footprint and high resolution. (B-D)In-situ computer vision captures actual UV intensity distribution on theresin surface. Clear resins (B) scatter less UV light which results inuniform gradation of intensity on the resin surface (white→gray) whileheterogeneous resins (C), such as the photocurable latex, scatter moreUV light and lower the peak intensity distribution in the resin (graythroughout). (D) Normalization reveals non-uniform scattering inside theprojection area. (E) Comparing the desired dimension (red box) with thesquare pillar printed without compensating for the XY-UV scatterhighlights that the printed part exceeds design dimensions and hasrounded edges with poor edge definition (F) Compensating for XY-UVscatter through iterative optimization of projected intensitydistribution results in the fabrication of pillars with improveddimensional accuracy and edge definition. (G) Modeling the energydistribution for specimens printed via scanning process enables controlof cure-through and dimensional accuracy by varying scan speed andprojection frame rate. The optimized printing parameters selected forthis work are predicted to induce a XY-dimensional reduction of 8 μm ata cure depth of 100 μm. Truncating the cure depth by setting the layerthickness to 100 μm results in a cure profile that is similar to thedesired profile.

Our research demonstrates that photocurable latexes overcome theprintability-mechanical performance paradox, enabling VP of highmolecular weight polymers at a printable viscosity (<10 Pa·s); however,the existence of discrete colloidal particles introduces a new obstaclefor VP. The colloidal particles in the photo-reactive latex scatter thepatterned UV light, which is incident on the liquid surface.²³ Lightscattering (i) lowers the average intensity that the latex experiences,which lengthens cure times, and (ii) lowers the achievable printedfeature resolution and the surface finish of the printed parts. To thebest of our knowledge, a process-based approach to compensate for thisscattering does not exist in the literature, and therefore otherstypically employ UV absorbing additives in colloidal printingexamples.²⁴ These additives potentially disrupt colloidal stability andrestrict versatility of a latex printing approach. Thus, a new printingmethod is required, in concert with latex design, to mitigatelight-scattering effects. Our approach involves a computer vision-baseddetermination of printing parameters and enables precise fabrication ofcomplex geometric features from polymer latexes.

Mitigation of light scattering in heterogeneous photopolymers requires(i) imaging of the scattered intensity distribution on the resinsurface, (ii) prediction of resulting cured feature dimensions, and(iii) subsequent generation of corrected printing parameters (i.e.,exposure time and gray scaling of the projected layer bitmap) tocompensate for scattering effects and achieve target feature dimensions.Specifically, a machine vision device (i.e., digital camera) images aprojected UV test pattern and the scattered light around the projectedpattern at the resin surface. A computer vision algorithm then analyzesthe captured image to extract the intensity distribution of a singleprojected pixel from the captured test pattern. In conjunction with partdesign specifications (layer bitmap pattern and thickness) and materialcuring parameters (depth of penetration, D_(p), and critical energy,E_(c)), this computed single-pixel intensity profile quantifiesscattering effects in the resin and enables the use of our previouslydescribed VP process mode³⁰ to predict the cured feature dimensions forheterogenous polymeric systems in VP. The approach employs anoptimization scheme, and printing parameters are iteratively varied inthe model to maximize the fit of the predicted cured feature dimensionswith design specifications.

A digital camera captured the intensity map, I_(camera)(X, Y) for aprojected test pattern as a matrix with a relative intensity scaleranging from 0-255 (0=lowest intensity level, 255=highest intensitylevel). A computer-vision algorithm then extracted the intensitydistribution of a pre-selected edge pixel (I_(pix)(X, Y)).³⁰ Athresholding condition (Equation 1) enabled the computation of theextent and magnitude of spatial scatter (XY plane; i.e., resin surface)around the selected pixel. Inputting the captured single pixel intensitydistribution I_(pix)(X, Y) into our previously described irradiancemodel³⁰ numerically reconstructed the overall intensity distribution ofthe projected test pattern (I_(proj)(X, Y)), as shown in Equation 2.Normalizing and mapping the computed I_(proj)(X, Y) with the mean actualprojection intensity measured with a UV radiometer, facilitatedcomputation of the intensity levels in I_(pix)(X, Y). Compared tohomogeneous, clear systems, I_(pix)(X, Y) for the latex elucidatesextensive scattering and reduction in peak intensity, as evidenced bythe low intensity levels (gray) in the reconstructed image (FIG. 6B &FIG. 6C). Scattering caused the projected intensity to spread to aradius of 58 μm (FIG. 6C) from the center of the projected pixel, whichis 28 μm more than homogeneous, clear resins. Normalizing I_(pix)(X, Y)revealed non-homogeneous intensity distribution due to UV scattering bypolymer particles in the projection area, as shown in FIG. 7D.

I _(pix)(X,Y)=I _(camera)(X,Y), if I _(camera)(X,Y)≥I _(background)  (1)

where I_(camera)(X, Y) and I_(background) correspond to the intensitydistribution captured by the camera and the threshold intensity value(when the resin is not irradiated with UV light) respectively.

I _(proj)(X,Y)=ΣI _(pix)(x,y)×B _(i,j)  (2)

where i, j corresponds to the index of projected pixel in the bitmapimage (1≤i≤1920, 1≤j≤1080) and B_(i,j) is a discrete function thatrepresents the state of the pixel in the bitmap pattern (i.e., B_(i,j)=1if the pixel at location “On” and B_(i,j)=0 otherwise)

The previously developed energy and cure models³⁰ remained valid for thephotocurable SBR latex due to adherence to the Jacobs equation¹ fortarget layer thicknesses under 700 μm (FIG. 13 ). For the fabrication ofsmall parts (<50×35 mm) with the standard mask projection vatphotopolymerization apparatus, simulation of the energy profile andcomputation of cure width (I_(w)) for various exposure times (t)utilized the I_(proj)(X, Y) input and predetermined curing parameters,i.e., depth of penetration and critical energy, in the standard maskprojection energy and cure models as shown in Equation 3 and Equation 4,respectively.

E _(proj)(X,Y,Z)=E _(proj)(X,Y)×e ^(−z/Dp, for) 0<z<Z where  (3)

E_(proj)(X, Y)=I_(proj)(X, Y)×t, 0<t<T

l _(w)(Z)=x ₁ −x ₂ ,∀Z>0 where x ₁ =X when E _(proj)(X,0,Z)=E _(c) asX>0 and  (4)

x₂=X when E_(proj)(X, 0, Z)=E_(c) as X>x₁

Based on predetermined exposure time for a desired layer thickness (viathe Jacobs equation), a characterization specimen with square pillarswas fabricated. The printed pillar (FIG. 6E) and the simulations of theenergy profiles for the Schwarz lattice (FIGS. 17A-17B) for thephotocurable latex demonstrate poor edge definition and a disagreementbetween the projected cure width (l_(w)) and the design-specified width(l_(wd)) due to light scattering. An optimization scheme (Scheme 8)corrected this inaccuracy, as demonstrated in FIG. 6F, by iterativelyvarying the exposure time (t) and pixel gray-scaling ratio (p), whichenabled both gross and fine control of cure width, respectively. As anillustrative example, the utilization of this optimization scheme toprint a layer of the Schwarz primitive lattice from photocurable latexyielded an adjusted exposure time, gray-scaling ratio and layerthickness of 8 s, 0.7 and 129 μm, respectively. Specimens were printedwith a layer thickness of 100 μm to improve inter-layer networkformation.

For: 0 < t < T and 0 < p < 1 Solve: E_(proj) (X, Y, 0)${{\text{where:}{I_{proj}\left( {X,Y} \right)}} = {\sum\limits_{i,j}{p \times {I_{pix}\left( {X,Y} \right)} \times B_{i,j}}}},{\forall\left( {X,Y} \right)}$Subject to: |l_(w) − l_(wd)| ≤ 10⁻⁹ Scheme 8. Optimization scheme tomaximize dimensional conformance on the resin surface in the XY planefor static VP systems; i.e. line width on the resin surface (l_(w)) =design line width (l_(wd)). To enforce this condition, layer exposuretime (t) and pixel gray-scale ratio (p), are iteratively incremented andthe resulting energy profile (E_(proj) (X, Y, 0)) and the line widthl_(w) are numerically computed. The optimization ends when |l_(w) −l_(wd)| is less than the selected tolerance of 10⁻⁹ and the resultantexposure time (t) and gray-scale ratio (p) are used for partfabrication.

The inverse relationship between projection area and projection pixelsize necessitated the utilization of our previously developed scanningmask-projection VP mode (S-MPVP)^(4,30) for fabrication of large parts(>50×35 mm) such as the tensile dog bone specimens (FIG. 16 ). DuringS-MPVP, the projector scans across the resin surface (along the X axis)while projecting a movie (FIG. 7A) of patterned UV light with a framerate that is synchronized to the scanning velocity of the projector. Inaddition to increasing the printable XY scale, S-MPVP blends theintensity distributions of each pixel and the synchronized movie ensuresequal energy delivery (i.e., exposure time (t)) for each pixel locationon the part surface, thus eliminating the need for the complex grayscaling algorithms used in standard static mask projection VP systems.The total energy delivered (E_(proj)(X, Y)) to the resin is thecumulative spatial sum of the energy delivered during the scanning ofeach projected movie frame (E_(f)(X_(f), Y_(f))), as shown in Equation5. As before, computation of the energy profile inside the latex and thecured line width occurs according to Eqns. 3 and 4, respectively.

$\begin{matrix}{{E_{f}\left( {X_{f},Y_{f}} \right)} = {\frac{1}{v_{s}}{\int_{0}^{X + r}{{I_{f}\left( {x,Y} \right)}{dx}}}}} & (5)\end{matrix}$

where r is the distance travelled by the projector, with velocity v_(s),before the projector frame is updated.

${v_{s} = \frac{r}{t}},$

where t is the exposure time for a projected pixel. Thus, the frame rateof the movie (F_(rate)) then becomes

$F_{rate} = {\frac{1}{t}.}$

E_(proj)(X, Y)=E_(f1)(X_(f1), Y_(f1))+ . . . +E_(fn)(X_(fn), Y_(fn)),where X_(fn), Y_(fn) correspond to the local projection co-ordinatesystem for the n^(th) frame, where X_(fn)=X_(f(n-1))+r.

In an illustrative example, this approach simulated the fabrication of a300-μm wide pillar, with r assumed to be equal to the pixel pitch (30μm), for various exposure times. Like static VP scattering compensation,iterative variation of the exposure time (t) provided gross control toalign computed line width (l_(w)) on the resin surface with designspecifications (l_(wd)), shown in Scheme 9. However, S-MPVP introducedscan speed and frame rate as new print parameters, which, together withlayer thickness, were subsequently computed using the optimized exposuretime (FIG. 14 ). The simulation with optimized and non-optimizedparameters, FIG. 7G, show that the line width with optimized parametersmatches the designed line width and the cure profile resembles thedesired cure profile when the cure depth is physically truncated bycontrolling the layer thickness. The optimized printing parameters inthis work generated a cure profile with a XY dimension gradient of 8 μmfor 100 μm cure depth and a cure through of 50 μm. The flow-charthighlighting the process parameter generation for the S-MPVP process isshow in FIG. 14 .

For: 0 < t < T Solve: E_(proj) (X, Y, 0)${{\text{where:}{E_{f}\left( {X_{f},Y_{f}} \right)}} = {\frac{t}{r}{\int_{0}^{X + r}{{I_{f}\left( {x,Y} \right)}{dx}}}}},{\forall\left( {X,\ Y} \right)}$Subject to: |l_(w) − l_(wd)| ≤ 10⁻⁹ Scheme 9. Optimization scheme it tomaximize dimensional conformance on the resin surface in the XY planefor scanning VP systems; i.e. line width on the resin surface (l_(w)) =design line width (l_(wd)). Energy delivered to the resin in scanningmodel is controlled by the exposure time per pixel (t). Hence, t isiteratively incremented and the resulting intensity distribution foreach frame (I_(f) (x, Y)) is computed. Then, the energy delivered to theresin surface (E_(proj) (X, Y, 0)) and the line width l_(w) arenumerically computed for each iteration of t. The optimization ends when|l_(w) − l_(wd)| is less than the selected tolerance of 10⁻⁹ and theresultant exposure time per pixel (t) is used to compute the scan speedand the frame rate used for part fabrication.

While the S-MPVP system coupled with a computer-vision based processparameter generation enable fabrication of parts with high resolution,it was imperative that the UV-crosslinked green bodies demonstratedsufficient modulus to form self-supporting features, which withstanddrag forces experienced during a recoating process. Furthermore, toachieve high-speed, high-resolution fabrication, the manifestation ofgreen body strength must occur with a low UV exposure time (<10 s).G_(N) ⁰ serves as a metric for green body modulus, and irradiation timeto modulus crossover (G″/G′=1) gauges photocuring kinetics and aids inthe determination of printability for the latex. A resin compositionwith 0.4:1 PEGDA:NVP was experimentally determined as an optimalcomposition for printing (G_(N) ⁰=30 kPa, crossover time ˜1 s) whilemaintaining final sIPN ultimate strains above 500%. Printed greenbodies, such as the lattice depicted in FIG. 7A & FIG. 7B, demonstratesuccessful and accurate fabrication of positive (lattice struts) andnegative features (designed voids) throughout the bulk of the greenbody. This demonstrates that high-resolution features are achievablewith light altered from light-scattering photo-reactive polymericcolloids.

Evaluating Geometric Complexity and Elastic Performance of PrintedElastomers

Drying printed green bodies to yield elastic sIPNs results in the lossof a large volume fraction of water (˜45 vol. %), which is accompaniedby a commensurate, isotropic volumetric shrinkage of approximately 40vol. % (dimensional shrinkage of 15.6%). Literature suggests the uniquepromise of isotropic shrinkage as a mechanism for increasing theresolution of printed structures.^(4,31,32) Similar to our previousdrying procedures,⁴ slow isotropic drying on a porous substratepreserved structural fidelity of the complex geometric features, evenfor thicker objects (FIGS. 8A-8G). Shown in FIG. 15 , dry sIPN parts areoptically clear along the direction of fabrication. This optical clarityconfirms the absence of discrete interfaces between layers, whichcorroborates our previous work that attributes this feature tocrosslinking between each layer.⁴ Coalescence of particles both withinand across layers further aids this process.

3D printed tensile specimens (modified ASTM D-638 V) exhibit elongationsover 500% with an average stress-at-break of 9.7 MPa (FIG. 16 ), therebyrepresenting the first example of a 3D printed high-performance SBRelastomer. This approach successfully combined polymer performance withstructural precision, enabling the fabrication of mechanically robustand reusable elastic molds, as shown in FIGS. 8A-8G. Specifically, wedesigned and printed a mold for an impeller with undercuts (i.e.,profiles varying across X-Y-Z planes) to highlight an importantapplication of printed elastomers: soft molding of complex geometriesthat cannot be directly extracted from hard molds. As a proof ofconcept, an impeller was casted in a 3D printed SBR sIPN mold withField's metal (a eutectic alloy of bismuth, indium, and tin) andsuccessfully extracted from the soft mold without damaging eithercomponent (FIG. 8G).

TABLE 2 Summary of latex molecular weight and particle size acrosscompositional range Solids Particle wt % M_(n) M_(w) Content DiameterHMA (kg/mol) (kg/mol) D (wt %) (nm) 100 (PHMA) 147 955 6.51 48.2 113 8083.8 342 4.08 46.2 109 50 94.3 367 3.89 48.1 117 20 64.0 260 4.05 48.2104 0 (PMMA) 73.4 326 4.45 48.8 122

Conclusions

We report concurrent polymer and machine design to address the vatphotopolymerization (VP) printability-mechanical performance paradoxwith photo-reactive polymeric colloids (latex); we report the first-everprinted styrene-butadiene rubber (SBR) elastomer. The introduction oftunable photoreactivity into polymer latex and computer-vision-basedprocess parameter generation enabled VP printing of polymeric colloidsto yield mechanically strong and geometrically complex 3D geometries. Anunprecedented strategy for sIPN formation manifests the mechanicalproperties of the dispersed polymeric particles upon 3D coalescencethroughout the printed object without disrupting feature fidelity. Thiswork expands the opportunities for VP printing of elastomers withintricate features that exhibit extensibilities above 500%, nearly 200%above the leading commercial VP elastomers.³³ The tunability andmodularity of this approach, when combined with diverse scaffold andpolymeric particle compositions, suggests versatility beyond SBR latexesand elastomers.

Example 2. Polymer-Inorganic Hybrid Colloids for Ultraviolet-AssistedDirect Ink Write of Polymer Nanocomposites

Inorganic-polymer hybrid colloids present a modular and tunable route tofabricate polymer nanocomposites from low viscosity precursors; however,their use in additive manufacturing remains limited. This workintroduces 3D printable, photocurable hybrid colloids via theincorporation of continuous-phase photocrosslinking chemistry andwater-dispersible silica nanoparticles to polymer (styrene-butadienerubber, SBR) latex. Varied relative concentrations of polymer andinorganic particles allowed precise tuning of filler loading in thefinal nanocomposite and introduced a bimodal particle size distributionwith major rheological implications that enable extrusion-based 3Dprinting. Ultraviolet-assisted direct ink write (UV-DIW) additivemanufacturing of photocurable hybrid colloid pastes generatedfree-standing green bodies embedded with both SBR and silica particles.Subsequent drying of these green bodies allowed SBR particle coalescencethroughout the scaffold and around the silica particles, yielding ananocomposite semi-interpenetrating network (sIPN). Facile tuning ofsilica content in the hybrid colloid enabled tuning of both hybridcolloid rheological behavior and the mechanical properties of the finalsIPN nanocomposites to achieve 3D printing of silica-SBR nanocompositeswith ultimate strains above 300% and ultimate strengths of 10 MPa.

Materials

Carboxylated styrene-butadiene rubber (SBR) latex (Rovene 4176) wasgenerously donated by Mallard Creek Polymers Inc. The latex has a solidscontent of 50 wt % in water and a particle diameter range ofapproximately 120-170 nm. The SBR copolymer was approximately 50/50 byweight styrene and butadiene with a low level of carboxylic acid monomerneutralized with ammonia to provide colloidal stability. The polymercontains a high insoluble (gel) content from the polymerization processdue to intra-particle crosslinking during the polymerization process.

1-vinyl-2-pyrrolidinone (NVP), poly(ethylene glycol) 575 g/mol (PEGDA575), lithium bromide (LiBr), (3-aminopropyl)triethoxysilane, andsuccinic anhydride were purchased from Millipore Sigma and used asreceived. Methyl ethyl ketone (MEK), dimethyl phenylphosphonite and2,4,6-trimethylbenzoyl chloride were purchased from Alfa Aesar and usedwithout further purification. Colloidal silica nanoparticles (10-15 nm)dispersed in MEK (MEK-ST) were generously donated by Nissan ChemicalCorporation. HPLC-grade tetrahydrofuran (THF), dimethyl formamide (DMF),diethyl ether, and hexanes were purchased from Fisher Scientific andused without further purification.

Analytical Methods

Dynamic light scattering (DLS) was performed with a Malvern ZetasizerNano at 25° C. Thermogravimetric analysis (TGA) was performed with an TAInstruments Q500 at a rate of 10° C./min and an isothermal drying stepat 120° C. for 10 min. Tensile tests were performed with an Instron5500R equipped with a 200 lb load cell at 50 mm/min from dogbones thatwere both die-cut (ASTM D-638 V) from photocast films and 3D printed viadirect ink write (DIW) (ASTM D-638 IV, scaled proportionally to a 55 mmlength). Cyclic experiments were performed on the same Instroninstrument at a rate of 200%/min with a 30 s hold at 0% strain betweencycles. Dynamic mechanical analysis (DMA) was performed on a TAInstruments Q800 at 1 Hz, 3° C./min, and 0.2% strain. Rheologicalanalysis was performed on a TA Instruments DHR-2 rheometer with aconcentric cylinder geometry (28 mm bob diameter, 42 mm bob length, 30.4mm cup diameter) for both continuous flow and oscillatory experiments at25° C. An oscillatory time sweep experiment was performed to investigatethe recovery time of colloidal network structure to gauge wait timebetween experiments. Stress and strain sweeps were performed at aconstant frequency of 1 Hz. Photorheology was performed with the samerheometer equipped with a 20 mm parallel plate geometry with a UV lightguide attachment and OmniCure S2000 Spot UV Curing System light source.Unless stated otherwise, photorheological tests were performed at 0.2%strain, 1 Hz, and with a measured UV intensity of 250 mW/cm². Scanningelectron microscopy (SEM) samples were freeze-fractured in liquidnitrogen prior to imaging on a FEI Quanta 600 FEG utilizing theback-scattering detector and 20 kV accelerating voltage. Samples weresputter-coated with 7 nm of gold/palladium for imaging.

Synthesis of Lithium Acylphosphinate Photoinitiator (LAP)

Lithium acylphosphinate photoinitiator (LAP) was synthesized accordingto a previous procedure from literature.^(78,79) In a typical example3.00 g (17.5 mmol) of dimethyl phenylphosphonite was added to a 250-mLround bottomed flask and purged with argon for 15 min while stirring.3.20 g (17.5 mmol) of 2,4,6-trimethylbenzoyl chloride was added dropwisevia syringe to the flask while stirring and allowed to react 18 h. It isimportant to note that methyl chloride is a toxic, gaseous byproduct ofthis reaction and therefore the reaction purge outlet was bubbledthrough an aqueous ethanolamine trap. 6.1 g (70.1 mmol) of LiBr wasdissolved in 100 mL of MEK and the solution was added to the reaction.The reaction was subsequently heated to 50° C. for 10 min after which awhite precipitate formed. The reaction was then cooled and allowed torest at room temperature for 4 h to allow full precipitation. Thesupernatant was decanted, and the white powder precipitate was washedthree times with MEK. The LAP powder was then dried at room temperaturein vacuo overnight and stored in a sealed amber jar.

Surface Functionalization of Silica Nanoparticles

Surface functionalization of colloidal silica nanoparticles followedmethods described previously in literature.⁷⁰ In a typical example, 100g of a colloidal silica in MEK dispersion (25 g dry silica) was combinedwith 100 mL of THF in a sealed 500-mL round bottomed flask and purged 20min with argon while stirring vigorously. 3.258 g (14.7 mmol) of(3-aminopropyl)triethoxysilane was added to the flask. The reaction washeated to 60° C. and allowed to react for 16 h. The reaction dispersionwas subsequently poured into a series of 50-mL centrifuge tubes, eachdiluted 5× with hexanes, and centrifuged at 6,000 rpm for 5 min toprecipitate the particles. The supernatant was discarded, and theparticles were redispersed in THF. This purification process wasrepeated 3× before finally redispersing the amine-functionalizednanoparticles in 200 mL DMF in a 500-mL round bottomed flask and purgedwith argon for 20 min while stirring. 4.22 g (42.2 mmol) of succinicanhydride was dissolved in 10 mL DMF and added via syringe and thereaction was allowed to proceed for 12 h at room temperature. Theresultant carboxylic acid (COOH) functional particles were precipitatedfrom diethyl ether, centrifuged 3× (in a similar fashion as describedabove), and finally stored as a dispersion in ethanol for storage.Degree of functionalization, expressed as mmol COOH/g silica, wasdetermined via potassium hydroxide titration in ethanol.

Synthesis of Photocurable Polymer-Inorganic Hybrid Colloids

The design of all hybrid colloids targeted a constant total solids (SBRpolymer and/or silica) content in water and liquid scaffold precursors(NVP & PEGDA). The composition of the solids was then systematicallyaltered from 0% to 50% silica (0:100 to 50:50 Silica:SBR) withoutaltering the total solids content of the colloid. To these hybridcolloids compositions, a constant loading of scaffold precursors(NVP&PEGDA) was added (3.56:1 Solids:Scaffold, 2.5:1 NVP:PEGDA) whichwas experimentally determined to provide sufficient greenbody modulus.

In a typical example, a net solids content of 40 wt % solids in waterand scaffold precursors was targeted with a solids ratio of 50:50Silica:SBR, ie. 20 wt % each for the SBR polymer and silica.COOH-functionalized silica nanoparticles (NP—COOH) were precipitatedfrom stock ethanol solution by the addition of diethyl ether andcentrifugation at 6000 rpm for 5 min. The supernatant was decanted, andthe particles dried at room temperature in a vacuum oven to remove allsolvent. 2.25 g of dried nanoparticle was dispersed in a solution of3.16 g deionized H₂O, 0.903 g NVP, 0.361 PEGDA (575 g/mol), and 50.0 mgLAP by vortex and sonication until forming a viscous, clear dispersionwith an amber hue. 4.688 g of SBR latex was added to the dispersion andthe hybrid colloid was vortex until thoroughly mixed. Due to the highviscosity of high-silica hybrid colloids, the retention of bubblesproved an issue for the formation of pore-free photocured films andobjects. Light centrifugation (1000 g/mol, 2 min) enabled the removal ofbubbles from the paste-like colloids without causing sedimentation ofthe particles.

UV-DIW Printing Process

The Ultraviolet-Assisted Direct Ink Write (UV-DIW) printer consisted oftwo Zaber A-LST500 linear slides that provided the extruder 500 mm of inthe XY direction and a Zaber A-LST250 linear slide that provided thebuild plate 250 mm of travel in the Z direction. A Nordson EFD Ultimus VDIW System was responsible for extruding material and a KeynotePhotonics LC4500-UV Digital Light Processing (DLP) projector providedUV-irradiation at 405 nm with a measured intensity of 14 mW/cm2 on thebuild plate was responsible for curing the photosensitive ink. Theprojector is mounted adjacent to the extruder and can be turned on andoff which allows the entire layer to be extruded and then be curedhomogeneously at once. This ex-situ curing method eliminates nozzleclogging due to unwanted photocuring of the material at the nozzleexit.⁵⁰ As demonstrated, the latex ink had the appropriate rheology toretain its shape after deposition without the need for immediate UVcuring. However, by exposing each layer to UV irradiation for a fixedperiod after deposition was complete ensures the ink was not over curedand enough photocurable groups remained to form a strong interlayeradhesion with the next layer. Additionally, a homogenous cure likelycontributed to more isotropic material properties. The printer wascontrolled with a custom-built LabVIEW control software that usedstandard GCODE to control the printer's movements and turn the extrusionon and off.

Both ink formulations were printed using a stainless-steel nozzle withan inner diameter of 0.61 mm and 12.7 mm length supplied by Nordson EFD.Parts were printed onto glass substrates with a deposition speed of 4mm·s and each layer was exposed to UV irradiation for 15 s. 50:50Silica:SBR inks were extruded at a pressure between 41.4 and 48.3 kPa.30:70 Silica:SBR inks were extruded at a pressure between 10.3 and 17.2kPa. Extrusion pressures varied within the listed range due to slightvariability between batches. Due to evaporation of the continuous phase(Water and NVP), nozzle clogging was occasionally observed. Clogging wasreduced by purging the nozzle between prints at high pressures (+200kPa) for ˜5 seconds and frequently replacing nozzle.

Tensile bars consisted of 3×200 μm layers. Truss and Honeycombstructures were printed from 25×250 μm layers. Tensile bar extrusionroads were varied from 0, 45, and 90° orientation with respect toelongation direction and were spaced by 650 μm.

Post-Cure Processing (Drying and Particle Coalescence, Extraction)

All photocured green bodies were dried at 40° C. overnight in a vacuumoven to facilitate water removal for SBR particle coalescence and sIPNformation. Parts were then extracted in 5/95 v/v THF and water withsolvent exchanges at 2, 4, and 12 h. The solvent was then changed topure THF with solvent extractions at the same intervals to remove thewater. Extracted objects were then dried in a vacuum oven at roomtemperature overnight to remove THF.

Result and Discussion

FIG. 25 illustrates the strategy for 3D printing elastomernanocomposites via UV-DIW of photocurable polymer-inorganic hybridcolloids. Our previous investigations introduced the ability to 3D printlatex polymer colloids via photoactivated network formation in thecontinuous phase, which yielded a particle-imbedded hydrogel “greenbody”.⁶⁹ Subsequent water removal enabled semi-interpenetrating polymernetwork (sIPN) formation due to the coalescence of polymer particlethroughout the photocrosslinked scaffold. The localization ofphotochemistry in the aqueous phase of the latex affords uniquemodularity in particle selection, provided a preservation of colloidalstability. Incorporation of water-dispersible inorganic nanoparticlestherefore enabled a facile route to nanocomposites. Carboxylated silicananoparticles contain similar negative surface charges to carboxylatedstyrene-butadiene (SBR) particles and readily mixed with polymer latexwithout disrupting colloidal stability. The addition ofphotocrosslinkable network precursors into the continuous phase yieldedstable, photocurable hybrid colloids amenable to UV-based AM processes.Photocrosslinking and subsequent SBR coalescence throughout the scaffoldand around the silica nanoparticles yielded an elastomer nanocompositewithout disruption in geometric fidelity of the printed part.

Carboxylation of colloidal silica nanoparticles via facile silanechemistry⁷⁰ enabled dispersibility in aqueous media, illustrated inFIGS. 26A-26B. Condensation of (3-aminopropyl)triethoxysilane withsurface silanol groups yielded amine-functionalized nanoparticles.Subsequent nucleophilic ring-opening of succinic anhydride by the aminesgenerated surface-bound carboxylic acid moieties. Titration of thesurface acids confirmed a surface loading of approximately 0.350 mmolCOOH/g silica which corresponds to an approximate surface concentrationof 0.84 acid-groups/nm². Upon stoichiometric addition of triethylamine,the generation of negatively charged triethylammonium carboxylatemoieties enabled colloidal stability and dispersibility in water,evident by DLS measurements in FIG. 26B. Dynamic light scattering (DLS)confirmed nanoscopic dispersion of the silica in water, with a particlesize of approximately 12 nm.

Design of all colloids in this study focused on a constant solids (SBRand silica) content of 40 wt %, with the liquid continuous phaseconsisting of water and the scaffold precursors n-vinyl pyrrolidone(NVP) and poly(ethylene glycol) diacrylate (PEGDA, 575 g/mol).Systematic variation of the solids composition from pure SBR particles(0:100 Silica:SBR) to equal parts by mass of silica and SBR (50:50)yielded stable, photocurable hybrid colloids. The bimodal particledistribution introduced by the combination of silica nanoparticles (12nm) and SBR particles (140 nm) resulted in drastic rheological changesover this compositional range. At a given weight percent, the particleconcentration for small silica nanoparticles was significantly higherthan that for the much larger SBR particles. As a result, increasingsilica loading resulted in increased particle concentration whilemaintaining constant solids content in the colloid. Steady-shearrheological measurements, illustrated in FIGS. 27A-27B, elucidate themajor impact of this increased particle concentration on colloidviscosity.

Shear thinning behavior is a well-studied phenomenon in dispersesystems, including latex colloids.⁷¹ Typical investigations demonstratean increase in viscosity with volume fraction of solids (ϕ_(solids)),owing to increased interactions between particles as the colloid becomesmore concentrated.⁷¹ However in this work, the density of silica (˜2g/cm³) is larger than that of SBR⁷² (0.94 g/cm³), and thereforeincreasing the Silica:SBR mass ratio causes a decrease in ϕ_(solids)from approximately 0.39 to 0.32, for 0:100 and 50:50 Silica:SBR,respectively, despite a constant total solids mass fraction of 40 wt %.

To better understand the observed increase in viscosity (FIGS. 27A-27B)despite a decreasing ϕ_(solids), it is important to consult previousinvestigations into the rheology of bimodal particle distributions. Inbimodal systems with two discrete particle sizes at a constantϕ_(solids) and constant particle diameter ratio, viscosity increaseswith increasing relative fraction of small to large particles (beyond acritical value) due to a resultant increase in total particleconcentration and decrease in particle-particle distance. In otherwords, it requires more small particles to achieve the same overallϕ_(solids) compared to larger particles.^(73,74) In this work, the sizedifference between silica nanoparticles and SBR latex particles causesthe overall particle concentration to increase by a factor ofapproximately 400 from the pure SBR latex (0:100 Silica:SBR) to the50:50 Silica:SBR hybrid colloid. This provides an explanation for theobserved increase in viscosity with increasing Silica:SBR, shown inFIGS. 27A-27B, despite the decrease in ϕ_(solids).

VP is generally limited to printing resins with a maximum viscosity of10-15 Pa·s,^(75,76) thus precluding VP printing of a Silica:SBR ratioabove 10:90 (FIGS. 27A-27B). DIW successfully prints higher viscosityliquids that are unsuitable for VP and therefore provides an avenue for3D printing of the high-silica hybrid colloid pastes. During DIWextrusion, the nozzle applies shear rates of ˜50 s⁻¹, reducing theviscosity of the 30:70 ink to 0.5-0.6 Pa·s and of the 50:50 ink to 3-4Pa·s due to the large degree of shear thinning.

In addition to their shear-thinning behavior, the occurrence ofrheological solid-liquid transitions positions these hybrid colloids asuniquely ideal candidates for the platform. Oscillatory rheologicalmeasurements probed the storage (G′) and loss (G″) shear moduli of eachcolloidal ink over increasing shear strains and stress (FIGS. 28A-28B.)Generally, values of G′>G″ indicate “solid-like” properties which is anideal state for DIW ink to exhibit minimal flow and deformation upondeposition. Conversely, G″>G′ values indicate “liquid-like” behavior andflow, which is ideal for extrusion through the nozzle.⁵¹

Discussed previously, many DIW examples utilize the stress-induced,reversible liquid-solid transitions of yield stress/strain materials toinduce flow under a shear stress imparted by extrusion through thenozzle and to achieve subsequent solidification upon deposition, whenshear stress is removed, for retaining the as-depositedshape.^(51,56,60,63,64,67,68) Hybrid colloids with a bimodaldistribution enabled tuning of this phenomenon through relative particleconcentrations, as shown in FIGS. 28A-28B. At 30:70 Silica:SBR andabove, the colloids exhibited ideal yield stress rheological behaviorfor DIW. These colloids were solid-like (G′>G″) at low shear strains andstress; however, beyond a critical yield point, the colloidal structurerapidly disrupted, and the colloid undergoes a modulus crossover to theliquid-like state. Upon removal of this stress, the colloids rapidlyre-solidified. Silica to SBR mass ratios below 30:70 do not exhibit ashear yield stress and inks behave entirely as low-viscosity liquids(G″>G′) at all measured shear strains/stresses. This precluded them fromDIW selection due to their tendency to flow and spread upon depositionwhich makes high resolution features and subsequent layers impossible toachieve via DIW. Shear modulus systematically increased with silicacontent (particle concentration), and for the 30:70 and 50:50 Silica:SBRratios, critical yield stress/strains appeared which followed the sametrend. Unlike studies of filled systems which increase vol % filler toachieve yield stress behavior; these colloids decrease in vol % solidswith increasing Silica:SBR (with constant wt % solids). However, asdiscussed previously with viscosity effects, the increase in particleconcentration with the addition of the smaller silica nanoparticlessignificantly increases the number of charged surfaces and decreases theaverage distance between particles. This facilitates greaterparticle-particle interaction which yields stronger colloidal networks,evidenced by the greater shear moduli and higher yield stresses observedin FIGS. 28A-28B. It is important to note that because the rheologicalbehavior is primarily dictated by net particle concentration andrelative particle size, tuning of the latter parameter could enableadjustment of rheological behavior for either VP for DIW regardless ofdesired silica content (e.g. high-silica liquids for VP or low silicapastes for DIW), with the use of larger silica particles or smaller SBRparticles.

Equation 1 describes the calculation of maximum shear stress imparted bythe DIW nozzle during extrusion,

$\begin{matrix}{\tau = {\frac{\Delta P}{2L}r}} & (1)\end{matrix}$

where ΔP is the pressure applied for extrusion, L is the nozzle length,and r is the nozzle diameter.⁶¹ In this study a nozzle length of 12.7 mmand radius of 0.305 mm. The 30:70 Silica:SBR was extruded at 13.79 kPa,corresponding to a maximum shear stress of 165.6 Pa, well above itsshear modulus crossover stress of 7.5 Pa. Similarly, the 50:50Silica:SBR was extruded at 44.82 kPa, corresponding to a maximum shearstress of 538.1 Pa, well above its shear modulus crossover stress of 129Pa.

The timescale of the recovery of colloidal structure to its solid-likestate after the removal of shear stress is critically important forensuring shape retention upon deposition. To measure the time ofstructure yield and recovery, FIGS. 29A-29B illustrates an oscillatoryshear experiment that alternates between low (0.1%) and high (50%)strains that are below and above the yield point of both the 30:70 and50:50 Silica:SBR hybrid colloid compositions (FIGS. 28A-28B). The stepchange from high to low strain mimics the removal of stress when thematerial is extruded from the nozzle and records how quickly the networkstructure reforms to exhibit solid-like (G′>G″ crossover) behavior.

Both higher-silica hybrid colloids (30:70 and 50:50 Silica:SBR)exhibited rapid transitions between flow and solidification at high andlow strains, respectively, and consistent reproducibility of thistransition over multiple cycles. The G′/G″ crossovers occurred nearlyinstantaneously, suggesting that the inks would transition to solid-likebehavior soon after exciting the DIW nozzle and the extruded bead wouldexhibit minimal spreading. After crossover to solid-like properties, therecovery of storage modulus progressed over the course of minutes andapproached the original modulus exhibited before disruption within 3 minfor 30:70 Silica:SBR (FIG. 29A). Although also exhibitingnear-instantaneous crossovers, the 50:50 Silica:SBR colloid showedslower G′ recovery than 30:70 and did not reach the original modulusover the same timespan (FIG. 29B). This may be explained by thesignificantly higher modulus of the 50:50 colloid (approximately anorder of magnitude) which restricts mobility and slows the recreation ofcolloidal network structure.

While often utilized in lieu of these rheological transitions⁵³,photocrosslinking introduced a second, irreversible solidificationmechanism for the hybrid colloids, thus establishing a multi-facetedprocessing window for UV-assisted DIW. Photocrosslinking furtherincreased the strength of the deposited material, sufficient to supportthe weight of subsequently deposited layers. Additionally, the increasedstrength enabled handling of the greenbody and minimizes warping duringremoval from the printer for drying. FIGS. 30A-30B illustrates thephotoactivated crosslinking chemistry which produced a scaffold networkin the continuous phase around the particles and permanently solidifiedthe colloid. This “green body” state comprised a water-swollen hydrogenembedded with both SBR and silica particles. FIG. 30B detailsphotorheological measurements across all investigated Silica:SBR massratios. These measurements occurred at low oscillatory strains (0.2%),and therefore the higher-silica colloids displayed solid-like behaviorprior to UV irradiation at the 30-second mark. Upon irradiation, both G′and G″ rapidly increase, with liquid samples achieving permanentcrossovers within 5 s.

Observed in our previous investigations⁶⁹, drying of greenbodies invacuo enabled the SBR particles to coalesce with each other throughoutthe photocrosslinked scaffold, yielding a semi-interpenetrating network(sIPN). The loss of discrete mesoscale SBR phases decreased thescattering of visible light and the networks became translucent. FIG.30C shows this same occurrence for the unfilled sample, however thesilica-loaded sIPN's exhibited increasing opacity with silicaconcentration. Because the individual particles were too small toscatter visible light, evidenced by their formation of clear dispersionsin water, their opacity suggested silica aggregation to mesoscale orgreater dimensions which will be discussed further.

Designing hybrid colloids on a particle-by-particle basis, rather thanas composite or encapsulated particles synthesized via miniemulsion,enables precise tuning of composition and final filler content.Thermogravimetric analysis confirmed silica loading from 0 to 46 wt %(for 0:100 and 50:50, respectively) in the final sIPN nanocomposite,tuned through facile mixing of pure SBR and aqueous silica dispersions.Hybrid colloids comprised 40 wt % solids (silica and SBR), approximately10 wt % liquid scaffold precursors (NVP, PEGDA), and 50 wt % water.After water removal, the final sIPN comprised approximately 80 wt %Silica/SBR and 20 wt % photocrosslinked scaffold network. The latterremained constant across all colloid compositions and providedsufficient green body modulus for printing (˜10⁴-10⁵ Pa).

For this work, the allure of inorganic nanofillers centered on theirpotential to strongly direct and reinforce (thermo)mechanical propertiesof elastomers. By combining the modular tunability and processingadvantages of hybrid colloids with the functional effects ofnanofillers, this approach introduced the capability to processhigh-performance and functional materials at low temperatures and forcesand with the geometric complexity characteristic of 3D printing. Dynamicmechanical analysis (DMA) probed the thermomechanical effects ofnanosilica incorporation to latex-formed sIPN structures. Targetingdifferent silica concentrations enabled tuning of the reinforcement ofthe rubbery plateau tensile modulus (E′) by over three orders ofmagnitude. Our previous investigations of these sIPN's demonstratedshifting of the SBR glass transition temperature (T_(g)) due to phasemixing with the photocrosslinked scaffold through which itinterpenetrated.⁶⁹ The T_(g)'s for the nanocomposite sIPN's, as observedby a maximum in the tan δ, remained generally consistent with theunfilled sIPN, yet significantly broadened with higher silicaconcentrations due to increased interactions between the silicainterfaces the sIPN network.

Tensile analysis (FIG. 31B) confirmed ultimate strains above 300% forall sIPN's nanocomposites up to 30:70 Silica:SBR loading. However, thehighly filled 50:50 Silica:SBR composition displayed lower ultimatestrains and stresses which may be due to the large silica aggregatesformed at this composition (see FIGS. 32A-32H). Young's modulus andultimate stress also significantly increased with silica concentration.Previously, cyclic tensile experiments examined the reversibleelongation of the unfilled system which exhibited some plasticdeformation and strain softening attributed to the slippage ofuncrosslinked SBR chains and breakage of the photocrosslinked scaffold.Cyclic tensile experiments of 30:70 both at a static 100% strain maximum(FIG. 31C) and progressively increased (100%, 150%, 200%) maximumstrains (FIG. 31D) show significant softening of the composite observedas lower stresses upon reloading/unloading (Cycles 2,4,6) than duringthe first loading to each strain (Cycles 1,2,3). This observationclosely resembles the Mullins effect, which is a viscoelastic effectexaggerated by the presence and breakage of filler structures.⁷⁷

Scanning electron microscopy (SEM) of freeze-fractured surfaces offeredinsight into the size and distribution of silica particles throughoutthe sIPN. FIGS. 32A-32H contains SEM micrographs for loadings from theunfilled (0:100 Silica:SBR) to the highest filled (50:50 Silica:SBR)compositions. Imaging of backscattered electrons enabled strong contrastbetween the silica aggregates and polymer matrix. Energy dispersiveX-ray spectroscopy (EDS) provided elemental analysis to confirm thechemical identity of imaged structures based on relative concentrationsof carbon, oxygen, and silicon. As discussed previously, opacityincreased with silica loading which suggested aggregation beyond the 12nm diameter of individually dispersed silica nanoparticles. Silicaaggregates clearly appeared for all filled systems, with a trend towardlarger sizes for higher silica concentrations. SEM confirms the presenceof uniquely large aggregate at 50:50 Silica:SBR. However, both 10:90 and30:70 Silica:SBR exhibited evenly distributed microscale dispersions ofsilica. It is important to note that only the larger silica aggregatesare visible via SEM and therefore these micrographs do not preclude thepresence of individually dispersed silica nanoparticles. Because DLSdoes not provide evidence of micron-scale particles, silica aggregationlikely occurs during either the photocuring or drying/coalescencestages. The latter is a more probable explanation as coalescence andpenetration likely provide sufficient mobility and force to driveaggregation of previously dispersed silica nanoparticles. However, thismethod of sIPN formation from latex is unprecedented beyond our ownwork, and future investigations are necessary to better understand thismechanism.

As discussed, DIW printing focused on the 30:70 and 50:50 Silica:SBRhybrid colloid compositions due to exhibiting their appropriate shearyield stress behavior. In agreement with predictions based on therheological tests, both inks were extrudable at moderate pressures and,upon deposition, maintained their as-deposited shape, maintaining shapefidelity over the timescale required to print a single layer. UVirradiation then photocured the paste into a robust solid green bodycapable of supporting subsequent layers. Thus, DIW fabricatedthree-dimensional objects in a layer-by-layer approach which, upon waterremoval in vacuo, generated elastic sIPN nanocomposite geometries.

Unlike mask-projection VP, which fabricates entire layerssimultaneously, the extrusion-based approach of DIW yields interfaceswithin the x-y layers of objects in addition to the z direction(height). Therefore, the orientation of filaments within the x-y layerscan lead to anisotropy of mechanical properties due to the presence ofweak interfaces between filaments. The transition of latex-basedprinting to UV-DIW opens the opportunity to study the effect of polymerparticle 3D coalescence on process-induced anisotropy. FIG. 33Billustrates design and tensile curves for UV-DIW-printed dogbones withx-y interfaces at 0°, 45°, and 90° with respect to the tensiledirection. All specimens followed similar stress-strain profiles andexhibited comparable ultimate strains, which suggested minimal effect offilament direction for printed nanocomposite sIPN's.

CONCLUSIONS

Photocurable hybrid colloids present a modular and highly tunable systemfor 3D printing multi-component materials. Hybrid colloid design on aparticle-by-particle basis (rather than composite particles) allows forprecise, independent loading of inorganic fillers (silica) into thefinal nanocomposite, and the concomitant bimodal distribution stronglydirects the colloid's rheological behavior and accesses newpossibilities for AM platform selection beyond VP. Shear-dependentliquid-solid transitions in concert with irreversible continuous-phasephotocrosslinking generates a unique processing window for UV-DIWprinting which enables the fabrication of shape with subsequentphotocuring to generate robust, stackable green body layers. The fullimplications of this unique processing window remain to be explored infuture work. Upon water removal, SBR polymer particle coalescencethroughout the photocrosslinked scaffold and around silica nanoparticlesgenerates nanocomposites which exhibit well-dispersed silica aggregatesand significant reinforcement to (thermo)mechanical properties whileretaining high ultimate strains and reversible deformation. Isotropicpolymer particle coalescence throughout printed objects appears tomitigate concerns of anisotropy due to filament orientation. In sum, thedesign of photocurable hybrid colloids simultaneously introduced tunablecombinations of high molecular weight elastomer and silica nanofillerswith suitable processability for UV-DIW to enable the generation of 3Darchitectures of high-performance elastomer nanocomposites.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations, andare set forth only for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiments of the disclosure without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure.

We claim:
 1. A resin composition comprising i. a polymer colloidcomprising a discontinuous polymer phase comprising polymer particlesand a continuous solvent phase; ii. one or more photocrosslinkablescaffold precursors; and iii. a photoinitiator;
 2. The resin compositionaccording to claim 1, wherein the polymer particles comprise a polymerhaving a number average molecular weight of about 100 kg mol⁻¹ to about2000 kg mol⁻¹.
 3. The resin composition according to claim 1, whereinthe polymer particles comprise a dispersible polymer selected from thegroup consisting of polycarbonates, polymethacrylates, polystyrenes,polyamides, polyurethanes, poly(ethylene terephthalate), poly(lacticacid), poly(glycolic acid), polyhydroxbutyrate, polydioxanones,δ-valerolactone, 1-dioxepanones, polyesters, poly(ethylene glycol),poly(ethylene oxides), polyacrylamides, vinyl polymers, silk, collagen,alginate, chitin, chitosan, hyaluronic acid, chondroitin sulfate,glycosaminoglycans, poly(hydroxyethyl methacrylate),polyvinylpyrrolidone, poly(vinyl alcohol), poly(acrylic acid),polyacetate, polycaprolactone, poly(propylene, glycol)s, poly(aminoacids), copoly (ether-esters), poly(alkylene oxalates), polyamides,poly(iminocarbonates), polyoxaesters, polyorthoesters, polyphosphazenes,polypeptides and copolymers, block copolymers, homopolymers, blends andcombinations thereof.
 4. The resin composition according to claim 1,wherein the polymer particles comprise an elastomer.
 5. The resincomposition according to claim 1, wherein the elastomer is selected fromthe group consisting of natural rubber, polyisoprene rubber, styreniccopolymer elastomers, elastomers that include styrene-butadiene (SB)rubber, styrene-butadiene-styrene (SBS) rubber,styrene-ethylene-butadiene-styrene (SEBS) rubber,styrene-ethylene-ethylene-styrene (SEES) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-isoprene-styrene (SIS) rubber,styrene-isoprene-butadiene-styrene (SIBS) rubber,styrene-ethylene-propylene-styrene (SEPS) rubber,styrene-ethylene-ethylene-propylene-styrene (SEEPS) rubber, styrenepropylene-styrene (SPS) rubber, and others, all of which may optionallybe hydrogenated), polybutadiene rubber, nitrile rubber, butyl rubber,and olefinic elastomer, and copolymers and blends thereof.
 6. The resincomposition according to claim 1, wherein the polymer particles comprisea high T_(g) polymer such as a poly(arylether), polyester, a polyamide,acrylate polymers such as poly(methacrylate) and poly(methylmethacrylate), or copolymers thereof.
 7. The resin composition accordingto claim 1, wherein the polymer particles comprise a polymer having aT_(g) below a thermal degradation temperature of the photocrosslinkednetwork of the scaffold precursors.
 8. The resin composition accordingto claim 1, wherein the particles have an average diameter of about 50nm to about 500 nm.
 9. The resin composition according to claim 1,wherein the solvent phase comprises water or other aqueous solvents,organic solvents, or a mixture thereof.
 10. The resin compositionaccording to claim 1, wherein the polymer particles comprise alkenecontaining polymers selected from the group consisting of homopolymersand copolymers containing the monomers butadiene, isoprene,dicyclopentadiene (DCPD), ethylidene norbornene (ENB), vinyl norbornene(VNB), and chloroprene.
 11. The resin composition according to claim 1,wherein the polymer colloid comprises a surfactant.
 12. The resincomposition according to claim 1, wherein the polymer particles comprisepolymers having pendant acidic functionality some of which has beenconverted to ionic functionality via reaction with a base; or whereinthe polymer particles comprise polymers having pendant basicfunctionality some of which has been converted to ionic functionalityvia reaction with an acid.
 13. The resin composition according to claim1, wherein the photocrosslinkable scaffold precursors comprisecrosslinkable groups selected from the group consisting of hydrogenhalides, carboxylic acids, sulfuric acid, ammonium-containing compounds,carbonic acids, citric acid, acetic acid, and phosphoric acid.
 14. Theresin composition according to claim 1, wherein the photocrosslinkablescaffold precursors are in the continuous phase.
 15. The resincomposition according to claim 1, wherein the photocrosslinkablescaffold precursors are covalently attached to the polymer particles atan interface between the discontinuous polymer phase and the continuoussolvent phase.
 16. The resin composition according to claim 1, wherein aweight ratio of the polymer material to the scaffold precursor materialis about 2:1 to about 20:1.
 17. The resin composition according to claim1, wherein a weight ratio of the polymer material to the scaffoldprecursor material is optimized to maximize the polymer concentrationwithout disrupting the colloidal stability.
 18. The resin compositionaccording to claim 1, wherein the polymer colloid further comprisesinorganic particles selected from the group consisting of silica, carbonparticles, metal particles, ceramic particles, and a combinationthereof; and wherein the coalescence of the polymer results in theinorganic particles being encapsulated and dispersed within the polymer.19. A method of additive manufacturing of an article, the methodcomprising: a) photopolymerizing a resin composition according to claim1 to form a green body, wherein the green body comprises aphotocrosslinked network of the scaffold precursors having the polymerparticles entrapped and dispersed therein; b) drying the green body toproduce the article, wherein the drying results in penetration of thepolymer from the polymer particles through the scaffold and coalescenceof the polymer between the polymer particles.
 20. An article made by aprocess according to the method of claim 19.